Peter Brückner

Deep-Level Characterization in GaN HEMTs-Part I: Advantages and Limitations of Drain Current Transient Measurements

This paper critically investigates the advantages and limitations of the current-transient methods used for the study of the deep levels in GaN-based high-electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The article is divided in two parts within Part I. 1) We analyze how the choice of the measurement and analysis parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients, the duration of the filling pulses, and the method used for the extrapolation of the time constants of the capture/emission processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. 2) We present a database of defects described in more than 60 papers on GaN technology, which can be used to extract information on the nature and origin of the trap levels responsible for current collapse in AlGaN/GaN HEMTs. Within Part II, we investigate how self-heating can modify the results of drain current transient measurements on the basis of combined experimental activity and device simulation.

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

GaInN quantum wells grown on facets of selectively grown GaN stripes

Multiple GaInN quantum wells (QWs) were grown on facets with reduced piezoelectric fields (PFs) of selectively grown GaN stripes oriented along the ⟨11¯00⟩ and ⟨112¯0⟩ directions by metalorganic vapor phase epitaxy. We found a higher normalized growth rate for the GaInN QWs on the {11¯01} facets compared to the {112¯2} facets and the planar grown reference sample on unstructured template. The different luminescence wavelengths observed for the QWs on these different facets can partly be explained by the reduced PFs, but additionally indicate that the In incorporation efficiency depends on the facet type. On stripes with trapezoidal cross section, we found strong interfacet migration of In and Ga changing the local thickness and composition significantly.

Piezoelectric fields in GaInN∕GaN quantum wells on different crystal facets

Direction and strength of piezoelectric built-in fields of GaInN quantum wells have been experimentally determined. The quantum wells have been grown either on the conventional {0001} crystal plane of GaN or on {11¯01} facets of selectively grown GaN stripes. The emission peak position of the electric-field-dependent photoluminescence can be modeled and yields value and sign of the piezoelectric field dependent on the strain of the quantum wells. On the semipolar {11¯01} facets, the quantum wells show a much weaker field (−0.1MV∕cm) compared to quantum wells grown on polar {0001} planes (−1.9MV∕cm), consistent with theoretic predictions.

Polarized light emission from semipolar GaInN quantum wells on {11¯01} GaN facets

GaInN quantum wells with reduced piezoelectric field deposited epitaxially on the {11¯01} facets of selectively grown GaN stripes show distinct polarization properties of the emitted light. Systematic electro- and photoluminescence studies demonstrate that the light is linearly polarized parallel to the stripes, representing the ⟨112¯0⟩ direction of the GaN crystal. Our model calculations show that this is a consequence of the strain-induced valence-band splitting depending on the crystal orientation. The polarization ratio is calculated and compared to measured values.

Time- and locally resolved photoluminescence of semipolar GaInN∕GaN facet light emitting diodes

The authors investigate the carrier lifetime and photoluminescence (PL) intensity of a semipolar GaInN∕GaN sample which was realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes that have a triangular shape running along the ⟨112¯0⟩ direction. Time- and locally resolved PL measurements show drastically reduced lifetimes for the semipolar sample of only 650ps at 4K whereas lifetimes exceeding 50ns were found for a polar reference sample. Furthermore, more than a doubling of the luminescence intensity and a significantly reduced blueshift of the PL peak wavelength with increasing excitation power density provide further evidence for the presence of reduced piezoelectric fields in the semipolar sample.

Ultra-Wideband GaN MMIC Chip Set and High Power Amplifier Module for Multi-Function Defense AESA Applications

This paper presents measurement results of a monolithic microwave integrated circuit (MMIC) chip set and of an ultra-wideband high power amplifier (HPA) transmit module for multi-functional next-generation active electronically scanned antenna radar/electronic warfare/communication applications targeting the frequency range from 6 to 18 GHz. The reported chip set consists of a driver amplifier (DA) MMIC and an HPA MMIC on a high-power gallium-nitride process with high electronic-mobility transistors. The DA reaches a power gain of 11 dB and maximum output power of 2 W, which is sufficient to drive a final stage in a balanced configuration. The HPA reaches a typical output power of 12.5 and 10.6 W in pulsed and continuous wave (CW) operation, respectively. Measurements on the module level indicate 18.5-W typical output power in both pulsed and CW operation.

Improved GaN layer morphology by hydride vapor phase epitaxy on misoriented Al2O3 wafers

Crack-free thick GaN layers have been grown by hydride vapor phase epitaxy on on-axis as well as on off-axis GaN-Al2O3 templates. A dramatic difference in surface quality could be traced back to the misorientation of the substrates: Mirror-like layers have been obtained for slightly off-oriented substrates, whereas pyramids and other surface structures were found on samples grown on exactly oriented wafers. Such excellent surfaces may make further surface treatment prior to a subsequent use of these wafers in further epitaxial processes obsolete.

Microbeam high angular resolution x-ray diffraction in InGaN∕GaN selective-area-grown ridge structures

GaN-based 6-μm-wide ridge waveguides with InGaN∕GaN multiple-quantum-wells (MQWs) produced by metal organic vapor-phase epitaxy in the regime of selective-area growth have been studied with microbeam high angular resolution x-ray diffraction and reciprocal-space mapping. Variation of the strain from 0.9% to 1.05% and a factor of 3 for the thickness enhancement of the MQW period have been measured for different widths of the oxide mask surrounding the GaN-based ridges. Only when the trapezoidal shape of the ridge cross section is taken into account can the difference between the experimentally measured thickness enhancement and predictions of the long-range gas-phase diffusion model be reconciled.

Q- and E-band amplifier MMICs for satellite communication

This work discusses MMICs for the realization of spaceborn multi-Gigabit satellite communication systems. A broadband low-noise amplifier, based on a 50 nm GaAs mHEMT technology, has been developed for Q-band low-noise receivers. The amplifier shows a small-signal gain of 27.5 dB with a gain flatness of ± 1.2 dB and a noise figure below 2 dB over the entire targeted frequency range between 30 and 50 GHz. For the next generation of E-band transmitter modules, a GaN-based high-power amplifier with a small-signal gain above 15 dB between 70-75 GHz and a saturated output power exceeding 28 dBm at 74 GHz has been developed.