# Presentation: 938087

## Electronic, Chemical and Structural Properties of Metamorphic III-V Compound Heterojunctions & Devices: DMR-0313468Steven Ringel, Leonard Brillson, Wu Lu, John Wilkins Ohio State Univ.

- The objective of this interdisciplinary FRG is to explore the growth-property relationships of lattice-engineered compound semiconductor heterostructures.  By developing device-quality materials having lattice constants which can be “chosen” using metamorphic (lattice-graded) growth, unique electronic and optical properties leading to extremely high carrier mobilities, tunable bandgaps from the mid-infrared through the blue visible spectrum, among other properties are possible. Such materials will advance device technologies since properties can be designed to meet specific application criteria without constraints imposed by fixed lattice constants.
-  PANEL 1 shows how grading the anion component of InAsP buffers bridges the large lattice mismatch from an InP substrate to form a high quality virtual InAs substrate, resulting in a very high mobility InAs 2-DEG.  PANEL 2 shows the first InAsP-based composite channel high electron mobility transistor, to our knowledge. The fT is the highest among all types of composite channel HEMTs with the same gate length.

- Inset shows SEM micrograph of 0.25 m InAlAs/InGaAs/InAsP composite channel HEMT.  Typical small signal RF characteristics are shown.  Currently we have grown InAs composite channel HEMT using the results of panel 1, which promises fT moving toward THz for sub-100 nm gate lengths.

**PANEL 2**

- Very high 2-dimensional electron gas (2DEG) mobilities are observed using virtual InAs substrates with low threading dislocation densities (TDD) < 107 cm-2, upper right; lower right shows high TDD for direct growth of InAs on InP and the destruction of the 2DEG.

**n****2D**** = 1x10****12**** cm****-2**

**Notes:**
Aim of the Project: The specific goal of this project is to develop the materials science necessary so that III-V heterostructures that have lattice constants in between typical substrate materials (primarily InP to InAs) can be achieved with very low defect density so that the high carrier mobilities that are theoretically inherent in III-V alloys between these binary materials can be experimentally achieved, and thus facilitate electronic devices with substantially superior device characteristics over current technologies.   This requires achieving an understanding of how bulk and interface properties (structural, electronic, chemical, optical) depend upon materials growth parameters, and how in turn these manifest themselves in prototype devices.  This slide demonstrates the vertically integrated nature of this FRG team, from unique metamorphic, epitaxial growth, structural characterization and electronic characterization of metamorphic materials, to the use of such materials in early prototype transistors.
Results: Description and Impact: Panel 1.  This composite of 3 figures shows first a comparison of cross sectional TEM images taken of a fully relaxed InAs layer grown on a step-graded (metamorphic) InAsP buffer, where the As:P ratio is varied from 100% P (i.e. InP, to match the initial substrate), to 100% As (i.e. InAs).  Merging InAs onto a more common platform is of great interest as InAs possesses superior electronic properties as well as a bandgap well into the infrared that is of interest for various optoelectronic applications.  The key issue is how to bridge the > 4% mismatch in lattice constant and do so with a minimum of threading dislocation formation, a minimum of surface roughness and a minimum of buffer layer thickness.  As seen in the figure, very high quality InAs, which high-resolution XRD showed to be fully relaxed, was obtained using a step graded buffer of only 1.2 microns in thickness.  The threading dislocation density in the InAs layer is far less than 107 cm-2 (we are performing larger area measurements to obtain accurate numbers at the moment).  These structures exploit the advantages of an anion (group V) based compositionally graded buffer that we have shown to be superior over conventional cation graded buffers like InGaAs in a prior FRG project.  We believe that the final TDD measurement will show a value that is virtually identical to our previous low values (low-mid 106 cm-2) for lattice mismatched InAsP with a composition of only 40% As, for which the lattice mismatch is only ~ half of that shown here.  This suggests that dislocation motion (ie glide velocity) is substantially increased at high As content InAsP layers, thus showing that relaxed buffers at the larger lattice constants of these materials in fact becomes more efficient at relieving strain.  
The impact on electronic properties and quality is remarkable. Shown in this panel (lower right) is a X-TEM image of an InAs layer grown on InP without a graded InAsP buffer layer, and as seen a high density of threading dislocations are clearly evident.  The Hall effect measurement data made on InAlAs/InAs 2-dimensional electron gas structures shows extremely high mobilities for the AlInAs/InAs growth on the InAsP graded buffer, whereas for direct growth on InP not only are mobilities substantially reduced, there is also no evidence of forming the 2DEG.  To our knowledge, these are the first 2DEG structures ever reported in the InAlAs/InAs heterostructure system, the first 2DEG structure that has exploited the InAsP metamorphic buffer system, and are the highest InAs mobilities reported to date.  Note also the high sheet carrier density of 1012 cm-2 achieved in the InAs layer, meaning that very high frequency devices with reasonable power density may be possible.  
These results show how metamorphic growth has facilitated extremely high mobility structures by grading the lattice constant to a value at which extremely high speed electronic materials (InAs) can be accessed. Panel 2 shows an SEM image of a fully fabricated composite channel HEMT in which an InAsP layer, developed as part of our growth component in the FRG (Ringel), was used as a sub-channel (Lu) (This device is a precursor to incorporating the AlInAs/InAs 2DEG structure shown in panel 1.)  Our models suggest this to improve the breakdown and power performance of conventional InAlAs/InGaAs HEMTs. Due to the reduced conduction band offset at the InGaAs/InAsP interface compared to conventional InGaAs/InP interfaces, electrons can be more easily transfered to the sub-channel. The 0.25 m HEMTs we fabricated showed a fT of 115 GHz and fmax of 137 GHz. To our knowledge, this is the first report of a HEMT using InAsP as composite channel material, and the fT is the highest among any type of composite channel HEMTs with the same gate length. Currently, we are downscaling the gate length to sub-100 nm region which will push the fT higher than 200 GHz (we are in the midst of testing).  We have also grown a composite channel HEMT using strained InAs as a sub-channel which is awaiting processing.  This most advanced structure, which includes the metamorphic InAs of panel 1, when combined with sub-100 nm gate length will move device fT toward the THz regime, and this is the primary focus of device activities in the coming year.

##  Nanoscale electronic and chemical imaging of double heterojunctions using cross sectional electron microscopy to explore local electronic properties of complex interfaces

**500 nm**

**GaAs**

**InGaP**

**InGaP**

**Vacuum**

** **Nanoscale electronic and chemical imaging of double heterojunctions using cross sectional electron microscopy to explore local electronic properties of complex interfaces
-  Micro-cathodoluminescence and work function techniques reveal impact of atomic layering sequence on heterojunction band offsets

**Electronic, Chemical and Structural Properties of Metamorphic III-V Compound Heterojunctions & Devices:**** ****DMR-0313468 Ohio State Univ. *****Steven Ringel, Leonard Brillson, Wu Lu, John Wilkins***

**Notes:**
Aim of the project: This slide represents our ability to image directly the electronic properties of heterojunctions on a nanometer scale. Epitaxial thin film structures grown by the Ringel Group are cleaved in ultrahigh vacuum to expose clean, ordered cross sections of heterojunctions, then transferred into an adjoining ultrahigh vacuum scanning electron microscope.  The 5 nm spot size incident electron beam impinging on the cross sectional stack (pictured in the lower left cartoon) excites the constituent atoms, producing free electrons and holes that subsequently recombine and give off light. The energy spectrum of this light (lower right panel, yellow background) yields a measure of the semiconductor band gap, its alloy composition, phonons replicas, and any defects as a function of spatial position. The incident electrons also produce Auger transitions within individual atoms that escape the solid with energies characteristic of that atom’s atomic number and chemical bond strength. In addition, the absolute kinetic energies of these electrons reveal the potential at each spatial position, providing a measure of heterojunction band offset and interface band bending.
Results description: The upper right figure shows two types of data: (1) a secondary electron image (black & white) of the same epitaxial structure illustrated schematically in the lower left – an InGaP/GaAs/InGaP double heterostructure, each layer 500 nm, all grown lattice-matched to a GaAs substrate and (2) a logarithmic intensity plot versus distance on the same absolute length scale of two spectral features – the near band edge exciton luminescence peak of the GaAs and that of the InGaP.  There is a clear correspondence between the band gap emissions of these semiconductors and the layers pictured in the secondary electron image. Plotting the intensity linearly (not shown) vs. logarithmically would accentuate the abruptness of each interface transition and intensity variations within a given layer. However, the log plot reveals luminescence intensity extending away from the corresponding layer and illustrating an asymmetry in the diffusion length of carriers from the confined GaAs layer to either InGaP layer – 4.5 times more for InGaP grown on GaAs vs. for GaAs grown on InGaP.  This asymmetry has at least two physical causes: (a) the change in growth temperature (from 490 C to 565 C for GaAs on InGaP, from 565 C to 495 C for InGaP on GaAs) and (b) the appearance of sub-gap GaAs defect emission at 1.41 eV (see spectra above) that is strongest near the GaAs on InGaP heterojunction.  Heterojunction band offset measurements in progress will tell us whether this asymmetry in carrier diffusion and recombination is due primarily to a difference in band offset for GaAs on InGaP vs. InGaP on GaAs as opposed to diffusion lengths reduced by increased defect trapping.
Significance/Impact of the results: These are the first direct measurements that correlate electronic and chemical properties of semiconductor heterojunctions on a nanometer to growth processes. This combination of electronic and chemical measurements is unique in probing the role of interface atomic bonding on local charge transfer and heterojunction band offsets, particularly as a function of growth sequence, anion and cation transitions, and metamorphic crystal strain. Understanding the principles involved enables growth of structures based on new lattice constants that can advance microelectronic and solar cell technologies.
Future plans: Thus far we have used cross sectional cathodoluminescence spectroscopy measurements to explore the GaAs/InGaP/GaAs/InGaP structure described here, as well as GaAs/InAlP/GaAs/InAlP, GaAs/InAlP/InGaAs/InAlP, and GaAs/In0.15Ga0.85As/In0.63Ga0.37P/In0.15Ga0.85As structures. These measurements are intended to test the impact on electronic properties of switching anions versus cations across the heterojunction as well as the ability to grow metamorphic structures in new lattice parameter domains. We now plan to measure the chemical composition using Auger electron spectroscopy and secondary ion mass spectrometry plus the heterojunction band offsets for each interface in each of these 4 systems. These measurements will establish the correlation between interface growth and chemical reaction properties with heterojunction band offsets in these model systems. 

## Electronic, Chemical and Structural Properties of Metamorphic III-V Compound Heterojunctions & Devices: DMR-0313468Steven Ringel, Leonard Brillson, Wu Lu, John Wilkins Ohio State Univ.

- Five grad students (Yong Lin, Dongmin Liu, Mykola Bataiev and Phil Smith) and 5 undergraduate students (Andrew Carlin, Drew Malonis, Steve Barr, Kaden Hazzard and Joe Gorse), some with REU support,  contributed to this FRG program.  Carlin is now an OSU grad student who contributes to this FRG.
**Outreach**
- Nanotechnology Science Summer Camp: We have developed a “Nanotechnology Summer Camp” for middle school students by collaborating with Columbus Center of Science and Industry Museum. The summer camp emphasizes “hands-on” experience for students, who form teams to complete a science fair project under the supervision of graduate students.
- OSU-NSF K-12 Program: Through an NSF OSU K-12 Fellowship Program, one of the PIs (Lu) and graduate student Michael Schuette regularly visit 3rd-5th classrooms in a local public school regularly. The program aims to enhance the scientific learning for students in grades 3 through 5 by utilizing inquiry-based teaching methods and aids in teaching of science for educators.

**Electronic, Chemical and Structural Properties of Metamorphic III-V Compound Heterojunctions & Devices: DMR-0313468***Steven Ringel, Leonard Brillson, Wu Lu, John Wilkins **Ohio State Univ.*

- Two groups of high school students in the “Nanotechnology Summer Camp” working in the OSU High Speed Device and Circuit Lab led by Lu on a project to characterize high speed III-V transistors and circuits with help of graduate students**.**

- High school students in the “Nanotechnology Summer Camp” working in the OSU ECE Cleanroom on a project using optical lithography and ICP-RIE with help of graduate students.