Steps to the exploitation of millimeter and sub-millimeter wave generation and detection for communication, sensing, and imaging

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1 Steps to the exploitation of millimeter and sub-millimeter wave generation and detection for communication, sensing, and imaging

2 Collaboration: Germany: TU Darmstadt, Institut für Hochfrequenztechnik, Department of High Frequency Electronics (Prime Contractor), Prof. D. Pavlidis,, Prof. H. L. Hartnagel Forschungszentrum Juelich,, Prof. H. Lueth,, Dr. S. Viltusevich Ukraine: Institute of Semiconductor Physics (Prime Contractor), NASU, Prof. A. Belyaev,, Prof. V. Kochelap,, Prof. V.Litovchenko Litovchenko. Institute of Physics, NASU, Prof. B. Danilchenko SRI Orion,, Dr. N. Boltovets

3 Goals of the Project to explore efficient carrier response mechanisms in ultra-high frequencies to demonstrate their feasibility in the generation/detection of sub-millimeter electromagnetic waves in nanoscale nitride-based structures.

4 How to approach the goals? We will utilize the unique advantages of various electron transport regimes of the nitrides in a simple two-terminal terminal device configuration

5 Why nitrides?

6 Energy Gap vs Lattice Constant

7 Semiconductor material properties at 300K Property Si GaAs 4H-SiC GaN Bandgap E g (ev) Breakdown field E B (MV/cm V/cm) Electron mobility µ (cm 2 /V s) Maximum velocity v s (10 7 cm/s) Thermal conductivity χ (W/cm K) Dielectric constant ε

8 Semiconductor materials for rf applications

9 Gunn effect The NDC predicted for the nitrides and observed in GaN was used to propose a Gunn generator working in the regime of domain formation However, one can utilize an alternative regime with suppressed Gunn domain formation [i.e., the so-called limited space-charge charge accumulation (LSA) mode] determined by 1/(3τ M ) < ν < 1/τ m (τ m is the dielectric relaxation time calculated at pre-threshold fields). LSA Advantages: High Power/Efficiency & High Frequency Hence, generation of THz microwave may be achievable under the LSA mode in a properly designed nitride device.

10 Steady-state characteristics of bulk GaN Max V dr cm/s at F 153 kv/cm. Other features: a portion of I-V with a turn-up (the rudiment of the runaway effect), a sharp increase in the temperature (T e > 3150:::4200 K).), negative resistance.

11 High-frequency small-signal signal conductivity of hot electrons in nitride semiconductors (sub-threshold fields) (a) Real part and (b) imaginary part of the small-signal signal conductivity as functions of normalized frequency f/f 0 for GaN at diff fferent values of sub-threshold field (solid squares in the insets): 1 - ε = 0.13 (33 kv/cm), 2 - ε = 0.24 (63 kv/cm), 3 ε = 0.56 (144 kv/cm), 4 - ε = 0.59 (151 kv/cm). The insets show the steady-state state velocity (a) and electron temperature (b) characteristics, f 0 = 8.9 THz.

12 High-frequency small-signal signal conductivity of hot electrons in nitrides small-signal signal conductivity of the hot electrons shows a large and very fast response for the nitrides cut-off frequency of the electrical (Gunn) instability is in the THz frequency range (5.5 THz for GaN).. ε / ε 0

13 Drift velocity of 2D electrons in AlGaN/GaN heterostructure 10 7 Drift Velocity (cm/s) K 4.2K Theory Electric Field (kv/cm)

14 Self-heating effect on conductivity of 2D channel 150 I, ma Sapphire 30ns 1µs dc SiC dc E, kv/cm

15 Streaming transport The nitrides provide another interesting possibility for electrically cally pumped sub-millimeter wave generators. The approach is based on strong electron-optical optical phonon coupling and large optical phonon energy characteristic for the nitrides. When the coupling is strong, the electron motion may become nearly periodic if optical phonon emission is the dominant scattering mechanism. Indeed, in an appropriate range of the dc electric field, an electron accelerates quasi-ballistically until it reaches the optical phonon energy ħω op. Then, the electron loses its energy by emitting an optical phonon and starts the next period of acceleration. This periodic motion [often called the optical-phonon phonon transit-time time (OPTT) resonance] provides the operating principles for electrically pumped sub- millimeter wave sources and has been observed experimentally in InP only at low temperatures

16 Transport at low electron concentrations (idealized picture) If electrons s have individual moment and energy balance, their motion becomes nearly periodic at low crystal temperatures,, when opt.. ph. p emission is a dominant scattering mechanism. In n an appropriate range of the dc electric field, an electron accelerates quasi- ballistically until it reaches the opt. ph. energy. The electron then loses its energy by emitting an opt. ph. And starts the next period of acceleration. This results in an anisotropic, streaming- like steady-state state distribution. Passive Region E< Ћω V Ћω ε ε Active Region E> Ћω t

17 THz-frequency resonances and negative dynamic conductivity of two-dimensional hot electrons in group-iii nitrides (I) Optical phonon transient time resonance in 50 A GaN QW at 77 K and F= 1.87 kv/cm Real and imaginary parts of microwave mobility as functions of the frequency. The fundamental resonance at 0.44 THz The second resonance at 0.88 THz Near first two resonances there are frequency windows with negative microwave mobility: The first band 0:41:: 1:::0:52 :52 THz. The second band 0:84:: 4:::0:96 :96 THz. In the inset: microwave mobility as a function of the field at ν = 1.09 THz

18 THz-frequency resonances and negative dynamic conductivity (2) The fundamental OPTTR resonance at F = 4.5 kv/cm ν = 1.09 THz Window of negative microwave mobility: THz

19 Conclusions -1 Band structure and electron-phonon interaction in nitride semiconductors bring a number of unique properties of high-field electron transport. These include: Very large peak drift velocities of steady state regime. Very large electron heating. A large overshoot effect (high transient velocity). Very fast response to a microwave field (a few THz cut-off frequency). Negative differential conductivity.

20 Conclusions -2 In short diodes (<1000 Ǻ) ) transient time below 0.1 ps and cutt-off frequency up to 1 THz is expected. At low electron concentrations the streaming transport regime and OPTT resonance may occur in both 3D and 2D structures. For nitride based QWs OPTT resonance is realized in modest electric fields (1 5 5 kv/cm) for frequency range THz. Considerable negative microwave mobility can be reached in THz windows near OPTT resonances at 77K and higher temperatures. We suggest that an electrically pumped THz laser operating above the nitrogen temperature can be achieved by using the streaming effect in 2D electron gas in nitride heterostructures.

21 PROPOSED RESEARCH Task 1: Sub-millimeter emission based on the high field transport (Gunn effect, etc) We will concentrate primarily on the LSA mode with suppressed Gunn domains

22 PROPOSED RESEARCH Task 2: Sub-millimeter emission based on the streaming regime We e will design an optimal structure for the OPTT- based THz generation

23 PROPOSED RESEARCH Task 3: Resonant environment and sub-millimeter microwave resonators We e will analyze different methods to impose resonant conditions for frequency selection and tuning. These will include: traditional microwave resonators on waveguides, microstrip resonance transmission lines, quasi-optical cavities, and surface-plasmon resonators.

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26 THE PROJECT CONSORTIUM Technische Universität Darmstadt - overall co-ordination; ordination; - provision of MOCVD and MBE-grown AlGaN/GaN heterostructures,, including non-polar structures, and resonant tunnelling structures; - processing of devices using dry and wet etch methods and contacting; - electrical transport, C(V), optical X-ray X and TEM characterisation of structures and devices. - Initial high frequency characterization of lateral and vertical transport devices;

27 High Frequency Electronics at TUD Professor Dimitris Pavlidis GaN Devices and Nanostructures for High-Power, High-Frequency Applications and Sensors MOCVD Grwoth of GaN Materials High Electron Mobility Transistors (GaN( GaN) ) and Diodes Heterostructure Components for High Frequency Communications InP-, GaAs- and GaAsSb-based based HBTs MMICs with HBTs and HEMTs Tunable Optical Receivers, lasers and filters Microwave Monolithic integrated Circuits (MMICs( MMICs) InP- and GaN-based HEMT MMICs Power Electronics with III-Nitride devices Tunable MMICs with Ferro-,, Semiconductor-components components Terahertz Technology for Sensing and Communication THz signal Generation with Traditional and Widebandgap Semiconductors Advanced Electronic Materials and Components Spintronics III-V/III V/III-Nitride Sensors and Integrated Solutions Typical HBT structure GaN device Micromachined Cavity Integrated T-Ray - MEMS Components Biomedical Applications T-ray III-V Components and Systems Optoelectronic MEMS Probes

28 HFE/TUD Facilities Professor Dimitris Pavlidis Material Growth (MOCVD, MBE) Material characterisation (Hall, PL, XRD etc.) Lithography and Microscopy Cleaning and wet / dry etching processes Thermal processes (RTP, RTA) Evaporation (e-beam, thermal) and sputtering Post-processing (dicing, etc.) Measurements and Characterisation (DC, microwave, etc.)

29 THE PROJECT CONSORTIUM Forschungszentrum-Juelich - high frequency characterization of lateral and vertical transport devices including fabrication, testing and analysis of GaN nanowires; - optimisation of characteristics of devices and circuit components aimed towards a reduction of the noise level; - low-temperature (down to 30 mk) ) and high magnetic field characterisation (up to 14 T) of transport properties and capacitance-voltage (C-V) measurement; - the evaluation of excess noise sources by measuring spectral density of noise, comparing with quantum 1/f theory and with its prediction for the resulting phase noise in various systems.

30 THE PROJECT CONSORTIUM ISP-Kiev - complex electro-physical investigation and testing of lateral and vertical transport in nitride heterostructures and devices; investigation of radiation effects; - investigation of the physics of electron fluctuations in semiconductor nanostructures related to a non- equilibrium steady state; - simulation of equilibrium electric characteristics of conductive channels in nitride heterostructures taking into account the specific features of their formation.

31 THE PROJECT CONSORTIUM Institute of Physics-Kiev - measurements of the fundamental physical properties of structures and devices using magnetotunnelling spectroscopy, CW, ns-time resolved and ultrafast spectroscopy and phonon transport; - studies of non-linear electron transport and non- equilibrium fluctuation phenomena

32 THE PROJECT CONSORTIUM Orion -Kiev The high-frequency laboratory at SRI Orion Orion will be involved in the high-frequency studies in the course of the project. Specifically, optimisation of the contact systems and resonant environment for microwave study will be carried out.

33 Conclusions -Nanosize structures based on III-Nitrides are promising for millimeter and sub-millimeter wave generation and detection for communication, sensing, and imaging. -The Ukraine-German consortium involved in this Project has the necessary expertise and capabilities to perform the proposed work.

34 Possibility to control the separation between central and satellite valleys and thus NDC through nanoostructuring. TRANSFORMATION of the ELECTRON BAND STRUCTURE UNDER TRANSITION from BULK (SOLID LINE) to QUANTUM-SIZE (QS) STRUCTURE (DOTTED LINES) for Si (on the left) AND GaAs (on the right) GaAs