W. A. Doolittle

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.

Highly luminescent, high-indium-content InGaN film with uniform composition and full misfit-strain relaxation

We have studied the properties of thick InxGa1−xN films, with indium content ranging from x ∼ 0.22 to 0.67, grown by metal-modulated epitaxy. While the low indium-content films exhibit high density of stacking faults and dislocations, a significant improvement in the crystalline quality and optical properties has been observed starting at x ∼ 0.6. Surprisingly, the InxGa1−xN film with x ∼ 0.67 exhibits high luminescence intensity, low defect density, and uniform full lattice-mismatch strain relaxation. The efficient strain relaxation is shown to be due to a critical thickness close to the monolayer range. These films were grown at low temperatures (∼400 °C) to facilitate indium incorporation and with precursor modulation to enhance surface morphology and metal adlayer diffusion. These findings should contribute to the development of growth techniques for nitride semiconductors under high lattice misfit conditions.

Heat dissipation in high-power GaN electronics on thermally resistive substrates

The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3/spl omega/ technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.

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...

GaN betavoltaic energy converters

Semiconductor betavoltaic converters use energy from radioisotope sources to generate electricity for remote applications requiring power for 5-50 years. To be competitive with thermoelectric devices, they must achieve an efficiency above 20%. This paper presents the design rules and efficiency calculations for such high efficiency GaN betavoltaic converters, and experimentally demonstrates the radiation tolerance of GaN.

Comparison of Interfacial and Bulk Ionic Motion in Analog Memristors

Analog LiNbO2 memristors are characterized using potentiodynamic electrochemical impedance spectroscopy. It is shown that LiNbO2-based devices exhibit analog memristive and meminductive or memcapacitive behavior depending on the applied frequency. The impedance spectra are fit to a circuit model whose elements correspond to ionic and electronic effects in the bulk and at the electrode-semiconductor interface. By separating out the bulk and interfacial effects, it is shown that the majority of the ionic movement in the cation-based analog memristor is at the interfaces. It is also shown that ionic capacitance results from a drift field-driven phase shift between the ac bias and the ionic motion, whereas the ionic inductance phase shift results from diffusion relaxation. Thus, to maximize resistance changes due to this ionic movement, analog memristors should be fabricated with negligible bulk dimensions.

A novel computer based pseudo‐logarithmic capacitance/conductance DLTS system specifically designed for transient analysis

A new inexpensive, simple to construct, PC based deep level transient spectroscopy (DLTS) system has been developed that efficiently digitizes and analyzes capacitance and conductance transients by conventional DLTS methods (boxcar, rectangular, and exponential) as well as by several transient methods (nonlinear least squares, modulation function methods, and correlation method of linear predictive modeling). A unique ‘‘pseudo‐logarithmic’’ sample storage scheme allows each transient to be sampled at more than 11 different rates, permitting 3 to 5 decades of time constants to be observed in one thermal scan allowing the resolution of closely spaced defect energy levels. The high system flexibility allows weighting times (or transient observation times) to be selected between <200 μs to 3 days. This is the first report that details the merits, including faster response and recovery times, of using the new Boonton 7200 capacitance meter in a DLTS system. This is also the first reported use of the pseudo‐log...

High resolution x-ray diffraction analyses of GaN/LiGaO2

Lithium gallate (LiGaO2) is gaining increasing attention as a potential substrate for the growth of the important semiconductor GaN. In order to better understand this material we have performed high-resolution double- and triple-axis x-ray diffraction analyses of both the starting LiGaO2 and GaN/LiGaO2 following epitaxial growth. A high-resolution triple-axis reciprocal space map of the substrate showed a sharp, well-defined crystal truncation rod and a symmetric streak of intensity perpendicular to q002, suggesting high structural quality with mosaic spread. Triple-axis scans following GaN growth showed (1) the development of isotropic diffuse scatter around the LiGaO2 (002) reflection, (2) the presence of a semi-continuous intensity streak between the LiGaO2 (002) and GaN (0002) reflections, and (3) a compact pattern of diffuse scatter around the GaN (0002) reflection that becomes increasingly anisotropic as the growth temperature is increased. These results suggest that LiGaO2 permits the epitaxial growth of GaN with structural quality that may be superior to that observed when growth is performed on SiC or Al2O3.