Sangbeom Kang

Heterogeneous materials integration: compliant substrates to active device and materials packaging

The drive for the heterogeneous integration of materials has led to significant advances in materials and device processing, and in the understanding of defect production and control during epitaxy. Heterogeneous integration is driven by microelectronic and packaging trends, and the advent of new materials, such as GaN, that do not possess native substrates. During the last decade, these objectives led to research in the development of compliant substrates. While the ideal compliant substrate concept and implementation may be flawed, this research has certainly advanced materials integration technology. This paper will provide an overview of recent results in compliant substrate experiments and interpretation, and the related advancement of materials and device integration and packaging deriving from some of this research.

GaN metal–semiconductor–metal photodetectors grown on lithium gallate substrates by molecular-beam epitaxy

The growth, fabrication, and characterization of ultraviolet metal–semiconductor–metal (MSM) GaN photodetectors grown on LiGaO2 by molecular-beam epitaxy are reported herein. GaN/LiGaO2 material with dislocation densities of approximately 108 cm−2 and x-ray diffraction (00.4) full width at half maximum of 75 arcsec results in MSMs showing high responsivity, 0.105 A/W, at a reverse bias voltage of 20 V at 308 nm, and low dark currents of 7.88 pA at a 60 V reverse bias. Given the etch selectivity of the GaN/LiGaO2 system and the excellent performance of these devices, GaN device integration onto alternative substrates appears promising.

MBE growth of high quality GaN on LiGaO2 for high frequency, high power electronic applications

Abstract Lithium gallate is receiving ever increasing attention as a possible candidate for III-nitride material growth due to its superior lattice match. This paper summarizes the recent, promising results from material growths on lithium gallate including aluminum, gallium and indium-nitride alloys. Structural, optical and electrical diagnostics are presented. The use of this material could result in significant advantages for use in power electronic applications. Specifically, the present material quality is in some ways, superior to all other current substrate technologies since very high quality, extremely thin nitride material can be produced. The areas for which lithium gallate still lags other technologies is also discussed. Issues that directly effect electronic devices, such as overcoming the limited thermal conductivity via heat pipes or substrate removal, the possibilities of achieving higher p-type doping and higher indium concentrations through the use of low growth temperatures, obtaining single polarity material with nearly no mosaic spread in grain orientation, along with issues of substrate cost are discussed.

Growth of GaN on lithium gallate substrates for development of a GaN thin compliant substrate

Since we have found that an entire substrate can be chemically removed in less than 5 min and since GaN is impervious to chemical etching, the GaN on lithium gallate (LGO) system is an excellent template (due to near infinite etch selectivity) for developing a thin film/compliant GaN substrate. Here we report on our efforts to grow GaN on LGO, including improvement of the atomic surface morphology using pregrowth pretreatments. We also report the first transferred thin film GaN substrate grown on LGO, transferred off of LGO and mounted on GaAs. With this approach, (InAl)GaN alloys can be grown on thin GaN films, implementing a “compliant” substrate for the nitride alloy system. In addition, the flexibility of bonding to low cost Si, metal or standard ceramic IC packages is an attractive alternative to SiC and hydride vapor phase epitaxy GaN substrates for optimizing cost verses thermal conductivity concerns. We have demonstrated high quality growth of GaN on LGO. X-ray rocking curves of 145 arcsec are sho...

Electrical and structural characterization of AlxGa1−xN/GaN heterostructures grown on LiGaO2 substrates

In this letter, we report on the properties of a AlxGa1−xN/GaN heterostructure grown on LiGaO2. A two-dimensional electron gas (2DEG) is observed with mobility of 731 cm2/V s at room temperature and 2166 cm2/V s at 77 K. A comparison of the structural quality of the heterostructure as determined by x-ray diffraction shows significant improvement in comparison to a similar structure grown on a sapphire substrate. Secondary ion mass spectroscopy analysis indicates that lithium diffuses into the GaN during growth. The concentration decreases by two orders of magnitude from the substrate to the surface in a 0.8 μm thick GaN film. The enhancement of the mobility of the 2DEG compared to that of electrons in a uniformly doped film is due, in part, to the proximity of the 2DEG to the film surface, where the Li concentration is lower. In addition, we believe that the surface roughness plays a role in the mobility of the 2DEG. Despite these extrinsic factors, the good conductivity of the 2DEG shows the promise of L...

Low Temperature Nitridation Combined With High Temperature Buffer Annealing for High Quality GaN Grown by Plasma-Assisted MBE

The effect of the initial nitridation of the sapphire substrate on the GaN crystal quality as a function of substrate temperature was studied. GaN layers were grown by plasma-assisted molecular beam epitaxy (MBE) on sapphire substrates nitridated at different substrate temperatures. A strong improvement in the GaN crystal quality was observed at 100 °C nitridation temperature. Symmetric (0004) and asymmetric (10-5) full widths at half maximum (FWHM) of the x-ray rocking curves were 136 and 261 arcsec, respectively. This compares to an x-ray rocking curve full width at half maximum of 818 arcsec (0004) for conventional MBE buffer conditions. For our conventional buffer conditions, sapphire substrates were exposed to a N plasma at temperatures above 500 °C for 10min and then 25~50nm buffers were deposited without annealing at high temperature. The low temperature nitridation also shows an enhancement of the lateral growth of the GaN, resulting in larger grain sizes. The largest grain size achieved was approximately 2.8μm, while the average grain size was approximately 2.4μm at 100 °C nitridation temperature.

Recent advances in III-nitride devices grown on lithium gallate

Recent advances in the growth of III-nitride materials and devices include: (i) The reduction of the near-surface threading dislocation density in GaN on lithium gallate (LGO) to 2 × 10 7 cm -2 . (ii) The demonstration of GaN, 50 × 130 μm, metal-semiconductor-metal (MSM) photodiodes with extremely low leakage current, 0.11 pA at 2 V and 7.9 pA at 60 V, and UV photoresponse at 308 nm and 20 V of 0.105 A/W. (iii) State of the art MSM devices have been successfully removed from the LGO substrate and attached to silicon wafers with no degradation in current characteristics. (iv) Demonstration of very thin, 0.7 μm HFET structures, grown at a rapid rate of 0.9 μm/h, with near state of the art room temperature 2DEG mobilities of 1365 cm 2 /Vs at a sheet charge of 9 x 10 12 cm -2 . (v) The elimination of substrate impurity diffusion by inclusion of gettering buffers has also been demonstrated.

Heterogeneous integration: from substrate technology to active packaging

Heterogeneous integration of dissimilar materials and devices is necessary for the continued advancement of electronic and optoelectronic systems. A range of processes has been developed in recent years that will enable system integration and advanced packaging. Herein, we outline our approaches towards heterogeneous integration.

Thin film GaN metal-semiconductor-metal photodetectors integrated onto silicon

Summary form only given. In this paper, metal-semiconductor-metal (MSM) photodetector dark current and responsivity results for the growth of GaN on lithium gallate (LiGaO/sub 2/) substrates is reported, as well as the heterogeneous integration of thin film GaN MSM detectors onto host substrates.