Pawel E. Malinowski

Record Breakdown Voltage (2200 V) of GaN DHFETs on Si With 2-

In this letter, we present a local substrate removal technology (under the source-to-drain region), reminiscent of through-silicon vias and report on the highest ever achieved breakdown voltage (V_BD) of AlGaN/GaN/AlGaN double heterostructure FETs on a Si (111) substrate with only 2-μm-thick AlGaN buffer. Before local Si removal, V_BD saturates at ~700 V at a gate-drain distance (L_GD) ≥ 8 μm. However, after etching away the substrate locally, we measure a record V_BD of 2200 V for the devices with L_GD = 20 μm. Moreover, from Hall measurements, we conclude that the local substrate removal integration approach has no impact on the 2-D electron gas channel properties.

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In this letter, we present a novel approach to enhance the breakdown voltage (<i>V</i>_BD) for AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs), grown by metal-organic chemical vapor deposition on Si (111) substrates through a silicon-substrate-removal and a layer-transfer process. Before removing the Si substrate, both buffer isolation test structures and DHFET devices showed a saturation of <i>V</i>_BD due to the electrical breakdown through the Si substrate. We observed a <i>V</i>_BD saturation of 500 V for isolation gaps larger than 6 μm . After Si removal, we measured a <i>V</i>_BD enhancement of the AlGaN buffer to 1100 V for buffer isolation structures with an isolation gap of 12 μm. The DHFET devices with a gate-drain (<i>L</i>_GD) distance of 15 μm have a V_BD > 1100 V compared with ~300 V for devices with Si substrate. Moreover, from Hall measurements, we conclude that the substrate-removal and layer-transfer processes have no impact on the 2-D electron gas channel properties.

Buffer Thickness by Local Substrate Removal

We made and characterized an X-ray detector on a 25-μm-thick plastic substrate that is capable of medicalgrade performance. As an indirect conversion flat panel detector, it combined a standard scintillator with an organic photodetector (OPD) layer and oxide thin-film transistor backplane. Using solution-processed organic bulk heterojunction photodiode rather than the usual amorphous silicon, process temperature is reduced to be compatible with plastic film substrates, and a number of costly lithography steps are eliminated, opening the door to lower production costs. With dark currents as low as 1 pA/mm2 and sensitivity of 0.2 A/W the OPD also meets functional requirements: the proof-of-concept detector delivers high-resolution, dynamic images at 10 frames/s, and 200 pixels/in using X-ray doses as low as 3 μGy/frame.

Silicon Substrate Removal of GaN DHFETs for Enhanced (<1100 V) Breakdown Voltage

In this paper, the performance of AlGaN extreme-ultraviolet (EUV) detectors is optimized by a combination of experimental results, TCAD simulations, and theoretical modeling. Using the verified thin-surface-barrier model, key issues in technology development are identified. A first conclusion is that reducing surface defects at the metal-AlGaN interface is found to reduce diode leakage considerably, hence improving detector sensitivity. Evaluating the benefit of a fingered Schottky contact results in a second conclusion, as a semitransparent fully covering Schottky contact is found to provide a good compromise between EUV sensitivity and reduced leakage. Both conclusions are supported by experimental results.

X-Ray Detector-on-Plastic With High Sensitivity Using Low Cost, Solution-Processed Organic Photodiodes

We show that an advanced cathode buffer design, consisting of bathocuproine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag, increases the short-circuit current of organic planar heterojunction cells and reduces the J-V slope at reverse voltages. We study the physical origin of these effects by measuring reflectivity, voltage dependent external quantum efficiency, and voltage dependent photoluminescence. Our findings suggest that the observed effects are mainly associated with a voltage dependent polaron-induced exciton quenching in the C60 layer. Finally, this improved cathode buffer design is applied to a diindeno[1,2,3-cd:1′,2′,3′-lm]perylene/C70 based cell, leading to a considerable planar heterojunction efficiency of 5.7%.

AlGaN Schottky Diodes for Detector Applications in the UV Wavelength Range

We report on the fabrication of aluminum gallium nitride (AlGaN) Schottky diodes for extreme ultraviolet (EUV) detection. AlGaN layers were grown on silicon wafers by molecular beam epitaxy with the conventional and inverted Schottky structure, where the undoped, active layer was grown before or after the n-doped layer, respectively. Different current mechanisms were observed in the two structures. The inverted Schottky diode was designed for the optimized backside sensitivity in the hybrid imagers. A cut-off wavelength of 280 nm was observed with three orders of magnitude intrinsic rejection ratio of the visible radiation. Furthermore, the inverted structure was characterized using a EUV source based on helium discharge and an open electrode design was used to improve the sensitivity. The characteristic He I and He II emission lines were observed at the wavelengths of 58.4 nm and 30.4 nm, respectively, proving the feasibility of using the inverted layer stack for EUV detection.

Improved cathode buffer layer to decrease exciton recombination in organic planar heterojunction solar cells

We report on a detailed investigation of the degradation of AlGaN/GaN Schottky diodes grown on silicon, submitted to high reverse-bias. The analyzed devices have a vertical structure; thanks to this feature, it was possible (i) to characterize the effects of stress by means of capacitance-voltage (C-V) measurements, therefore, identifying and localizing the trap states generated as a consequence of the stress tests; (ii) to accurately control the intensity and distribution of the electric field over stress time. Results indicate that stress induces an increase in the leakage current, which is well correlated to the increase of a new capacitance peak in the C-V characteristics. Based on experimental data and bidimensional simulations, degradation is ascribed to the generation of donor traps in the GaN buffer, close to the AlGaN/GaN interface.

Extreme ultraviolet detection using AlGaN-on-Si inverted Schottky photodiodes

We report on the first measurement results to obtain over 2 kV breakdown voltage (VBD) of GaN-DHFETs on Si substrates by etching a Si Trench Around Drain contacts (STAD). Similar devices without trenches show VBD of only 650 V. DHFETs fabricated with STAD technology show excellent thermal performance confirmed by electrical measurements and finite element thermal simulations. We observe lower buffer leakage at high temperature (100°C) after STAD compared to devices with Si substrate, enabling high temperature device operation.

Degradation of AlGaN/GaN Schottky diodes on silicon: Role of defects at the AlGaN/GaN interface

We present the first measurement results from hybrid AlGaN-on-Si-based Extreme Ultraviolet (EUV) imagers with 10 µm pixel-to-pixel pitch. The 256×256 backside illuminated Focal Plane Arrays (FPAs) were hybridized to dedicated Si-based CMOS Readouts (ROICs). The AlGaN active layer with 40% Al concentration provides an intrinsic rejection of wavelengths larger than 280 nm (solar blindness), together with enhanced radiation hardness (1). Sensitivity in Deep UV (DUV), Far UV (FUV) and Extreme UV (EUV) was verified using synchrotron radiation down to a wavelength of 1 nm.

Si Trench Around Drain (STAD) technology of GaN-DHFETs on Si substrate for boosting power performance

In this letter, we report on the fabrication of near-ultraviolet photodetectors based on gallium nitride (GaN) layers grown on a Si(111) substrate. Optoelectronic characterization was performed using front-side and backside illumination, the latter possible by locally etching the Si substrate under the detectors using reactive ion etching. The dark current after removal of the Si substrate decreased by two orders of magnitude to around 20 fA at -1 V for a 300-mum-diameter Schottky photodiode. Responsivity at the cutoff wavelength (370 nm) was equal to 35 mA/W for the backside illumination. Detection at smaller wavelengths was not possible due to a nonoptimized layer stack. These first results do however illustrate the potential of backside-illuminated GaN-on-Si Schottky photodiodes in 2-D UV imagers.