Kyoung-Keun Lee

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.

III-nitride integration on ferroelectric materials of lithium niobate by molecular beam epitaxy

Integration of III-nitride electrical devices on the ferroelectric material lithium niobate (LiNbO3) has been demonstrated. As a ferroelectric material, lithium niobate has a polarization which may provide excellent control of the polarity of III-nitrides. However, while high temperature, 1000°C, thermal treatments produce atomically smooth surfaces, improving adhesion of GaN epitaxial layers on lithium niobate, repolarization of the substrate in local domains occurs. These effects result in multi domains of mixed polarization in LiNbO3, producing inversion domains in subsequent GaN epilayers. However, it is found that AlN buffer layers suppress inversion domains of III-nitrides. Therefore, two-dimensional electron gases in AlGaN∕GaN heterojunction structures are obtained. Herein, the demonstration of the monolithic integration of high power devices with ferroelectric materials presents possibilities to control LiNbO3 modulators on compact optoelectronic/electronic chips.

III-nitrides on oxygen- and zinc-face ZnO substrates

The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ∼108cm−2, while a dislocation density of ∼1010cm−2 was obtained on the on-axis ZnO substrates.

Reproducible reflection high energy electron diffraction signatures for improvement of AlN using in situ growth regime characterization

Recently published methods that answer the previously unresolved critical issue of in situ growth regime determination during molecular beam epitaxy of AlN are used to address issues of material quality and intergrowth nonuniformity for improved repeatability using a modulated flux technique. A shutter modulation growth technique, defined as metal modulation epitaxy (MME), using the previously published reflection high-energy electron diffraction (RHEED) signatures was developed with the goal of obtaining materials with the properties of droplet regime materials, without the adverse effect of droplets. The films grown using MME were compared to films grown with no shutter modulation, and the surface roughness determined by atomic force microscopy was improved. For an unmodulated sample without droplets, the rms surface roughness was 6.9nm, while a sample with droplets had a rms surface roughness of 1.2nm. For the same Al flux that resulted in droplets with the unmodulated sample, the MME sample had no dro...

The heterogenous integration of GaN thin-film metal-semiconductor-metal photodetectors onto silicon

The heterogeneous integration of GaN thin-film metal-semiconductor-metal (MSM) photodetectors onto a host substrate of SiO/sub 2/-Si is reported. Thin-film GaN photodetectors were separated from the lithium gallate (LiGaO/sub 2/) growth substrate using selective etching, and contact bonded onto a SiO/sub 2/-Si host substrate. The thin-film MSMs exhibited a dark current of 13.36 pA and an UV photoresponse at 308 nm of 0.11 A/W at a reverse bias voltage of 20 V. This first demonstration of GaN thin-film device integration onto SiO/sub 2/-Si using a low-temperature integration process, combined with the advances in GaN material quality on LiGaO/sub 2/ substrates, enables the integration of GaN devices with Si circuitry for heterogeneously integrated systems.

III-nitride growth and characteristics on ferroelectric materials using plasma-assisted molecular beam epitaxy

The characteristics and growth of III-nitride materials on ferroelectric lithium niobate with varying Li mole composition have been investigated to achieve a better device performance of AlGaN∕GaN heterojunction structures. III-nitride growth on lithium niobate has been performed after high temperature (1000°C) furnace thermal treatments in dry air environment resulting in atomically flat surfaces on lithium niobate (LN). However, while this furnace thermal treatment results in improved surface smoothness and III-nitride adhesion, it also causes repolarization, ferroelectric domain reversal from a +z spontaneous polarization to a −z spontaneous polarization in the surface of congruent LN (48.39mole% of Li2O). On the other hand, near-stoichiometric LN (49.9mole% of Li2O) did not develop repolarization during the identical thermal treatment. Furthermore, as determined in situ by spectroscopic ellipsometry, congruent LN (CLN) shows a bigger variation of the pseudorefractive index and pseudoextinction coeffic...

Growth and valence band offset measurement of PbTe/InSb heterojunctions

InSb (111)B and (211)B substrates were investigated for the growth of PbTe. The PbTe/InSb (111)B showed slip features caused by stain relaxation during cooling down, while growth of PbTe/InSb (211)B resulted in a better surface morphology. The valence band offset of PbTe/InSb (211)B was measured by high resolution x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy with the value of 0.38 ± 0.05 eV. The PbTe/InSb (211)B heterojunction structure forms a type-III—broken gap heterostructure with the conduction band offset of 0.23 ± 0.05 eV.

Design of a smart pixel multispectral imaging array using 3D stacked thin film detectors on Si CMOS circuits

We discuss the design of a smart multispectral imaging system which can be fabricated with independently optimized detector arrays and Si CMOS circuits, which are stacked in three dimensions on top of one another for complete physical registration of the multispectral image. This smart multispectral imager utilizes the heterogeneous integration of thin film GaN (UV), MCT (MWIR), and Si CMOS circuitry to create an outstanding example of the power of heterogeneous integration for enhancing optical systems using electronic signal processing capabilities.

Power-performance trade-offs in second level memory used by an ARM-like RISC architecture

Power consumption is an important dimension in microprocessor and digital system design. This is especially true in the embedded setting where microprocessors have to operate without the luxury of a large power supply or cooling structures. In this paper, we describe an infrastructure setup for the study of power-performance tradeoffs in microprocessor architecture and compiler optimizations. This infrastructure distinguishes itself from those already proposed in the literature in its use of power estimations based on synthesis of the architecture and the full integration of a well-established optimizing compiler framework. We present some preliminary results where we show how the circuit level and architectural techniques can be combined to save overall system power. In particular we reduce the clock frequency and supply voltage of level two memory accesses (circuit level technique) and compensate for the resulting increase in the completion time by implementing a non-blocking store buffer (architectural technique) resulting in up to 39 % less power and up to 28 % less energy on a set of candidate benchmarks.

Band structure measurement and analysis of the Bi2Te3/CdTe (111)B heterojunction

The band alignments of the Bi2Te3/CdTe (111)B (Te-terminated) heterojunction were investigated using high-resolution x-ray and ultraviolet photoemission spectroscopy. The measured valence band offset and the conduction band offset of the Bi2Te3/CdTe were 0.22 ± 0.05 and 1.12 ± 0.05 eV, respectively, and indicated that the Bi2Te3/CdTe heterojunction had a type I band structure.