2.1.

Importance of GaN in Plasma HFETs

III-N two-dimensional electron gas (2DEG) has exceptional transport characteristics. As a result, GaN-based devices have been proposed as promising advanced plasmonic electronic devices for detection, mixing, and generation of THz radiation.⁹⁵ Particularly, GaN provides exceptional characteristics for implementation of plasma HFETs. The fundamental plasma frequency is given by

As it is indicated in Fig. 4, ${\omega }_{\mathrm{cr}}$ happens to be in the THz regime for GaN for the entire temperature of operation range.⁹⁶ An FET operating in a conventional regime and in a plasmonic regime using the propagation of the wave of the electron density is shown in Fig. 5.

Fig. 4

Material critical frequencies of operation.

2.2.

History of GaN-Based Devices for THz Applications

With the first reported direct determination of a laser frequency in the far infrared⁹⁷ and optical frequency shifting of a mode-locked laser beam⁹⁸ both in 1967, generation and detection of electromagnetic radiation in THz frequency range are not older than 5 decades. The utilization of GaN for THz application is even younger with the first proposed AIGaN/GaN HFET for detection of THz radiation in 1997.⁹⁹ Figure 6 shows the first proposed measurement set-up of this GaN HFET in 1997. Karl-Suss Microwave Probe Station was used as the platform of the device. Two GGB microwave probes are used in the setup. The microwave power is fed to the GaN-HFET through one of the GGB microwave probes while another GGB microwave probe connects the drain of the device to the broadband bias tee. Figure 6 shows the detector responsivity for the proposed GaN HFET with the gate length $L=5\mathrm{pm}$, the gate bias $U_{\mathrm{G}}=-1\mathrm{V}$, and the threshold voltage $U_{\mathrm{T}}=-2\mathrm{V}$. Achieving responsivity up to 0.02 THz using GaN-HFET in nonresonance mode lead to the first predictions that operating frequencies of these devices could be pushed into the THz range of frequencies in order to use them as detectors, mixers, and sources. The photoresponse of these devices in nonresonance mode was improved to 10 mv for up to 0.6 THz in room temperature by 2002.¹⁰⁰

Fig. 5

An FET operating in (a) a conventional regime and (b) a plasmonic regime using the propagation of the wave of the electron density (plasma waves). Redrawn from Ref. 93.

2.3.

Implementation of Resonance-Mode HFETs for Detection of THz Radiation

The responsivities of the mentioned HFET in Sec. 2.2 exceed that of the Schottky diodes, which have been conventionally used as detectors and mixers in the THz regime.¹⁰¹ The HFET detectors work at two modes of frequencies. At the low-frequency mode, the HFET works in a nonresonant mode. The responsivity in this mode can be up to $600\mathrm{V}/\mathrm{W}$. The frequency in this mode of operation for the fabricated HFET is 0.05 to 20 GHz as Fig. 6 shows. At the high-frequency mode, which is typically much higher than the cutoff frequency, the HFET operates in a resonance mode. The resonance happens at the plasma oscillation frequency. This is due to the fact that when the electron density in the channel of HFET is high, instead of holding the 2DEG condition, 2-D electron fluid model holds. In a 2-D electron fluid, hydrodynamic equations analogous to water in a shallow channel are applicable. Plasma waves would appear in a similar manner that waves in the water appear. The velocity of the plasma waves depends on the gate bias and is higher than the electron saturation drift velocity. As a result, a new generation of devices with frequencies higher than the radio and microwaves could be realized. The peak responsivity in this mode is higher than that of the standard Schottky diode detector. The resonance peak appears at several THz in deep submicron devices. As a result, such AlGaN/GaN HFETs can work at frequencies much higher than the cutoff frequency and thus is a proper choice for THz applications. The mentioned phenomenon does not hold in GaAs HEMTs. As a result, GaAs HEMTs work at frequencies much lower than cut-off frequency^99,102 and were not proposed for THz applications. However, it needs to be mentioned that the original observations of infrared absorption¹⁰³ and emission¹⁰⁴ related to plasma waves in silicon inversion layers date back to 1977 and 1980, respectively. Although the operation of GaN-HFETs in resonance mode for THz applications was predicted in 1997, it was practically realized in 2006 with a maximum responsivity of $0.2\mathrm{V}/\mathrm{W}$ for 0.2 THz at room temperature.¹⁰⁵ Figure 7 shows the noise equivalent power (NEP) and responsivity against gate voltage of this device.

Fig. 6

(a) The measurement set-up of the detector using a GaN HFET. (b) Measured frequency dependence of the detector responsivity for a GaN HFET with the gate length $L=5\mu \mathrm{m}$, the gate bias $U_{\mathrm{G}}=-1\mathrm{V}$, and the threshold voltage $U_{\mathrm{T}}=-2\mathrm{V}$. The solid line shows the predicted dependence with $C=1.2$ to ${10}^{-7}\mathrm{F}/\mathrm{m}$, $(U_{\mathrm{GS}}-U_{\mathrm{T}})=1\mathrm{V}$, $\mu =0.1{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$. (c) The full data of (c) at frequencies between 0.05 and 2 GHz. Reprinted from Ref. 99, with permission of IEEE.

2.4.

GaN-Based Quantum Cascade Lasers

In the conventional semiconductor lasers, a photon is emitted as the result of the recombination of a conduction band electron with a valence band hole. The frequency of the emitted photon is equal to the energy difference between the conduction band and the valence band $E_g$ divided by the Planck constant $h$. The photon energy in the THz range is less than the energy gap between the conduction and valence bands of the active materials and thus for emitting THz photons, energy gaps with lower energies are needed. Intersubbands (ISBs) are introduced by realizing quantum wells (QWs). Through growing several repeated periods of QWs, usually by molecular beam epitaxy (MBE), THz-QCLs have been implemented. Not long after the first implementation in 1994 by a group of scientists including Alfred Y. Cho, known as the father of MBE, QCL became one of the most widely used approaches for generation of THz radiation.¹⁰⁶ The first THz-QCL was reported in 2002. It was realized by GaAs/AlGaAs heterostructures. It could emit high output powers of more than 2 mW at 4.4 THz. However, it could only operate at temperatures up to 50 K in pulsed mode.¹⁰⁷ The mentioned QCL could not operate in CW mode. By 2005, GaAs/AlGaAs-based QCLs have been improved to operate at 164 and 117 K in pulsed mode and CW, respectively.¹⁰⁸ However, operation of GaAs-based QCLs are limited by LO phonon of 36 meV. LO phonon for GaN is located at 92 meV.⁸⁷ According to the studies by the research group of Paiella and Moustakas at Boston University, as shown in Fig. 8, the population inversion and hence the gain coefficient of the GaN/AlGaN QWs dependence on the temperature is three times smaller than that of GaAs/AlGaAs for THz emission and the gain coefficient of the nitride device remain large enough for laser action even without cryogenic cooling.^109–111 As a result, GaN-based devices can operate in the upper THz frequency of 5 to 10 THz at room temperature, which is inaccessible by GaAs-based devices.⁸⁶ In 2003, pioneer works reported THz emission from InGaN/GaN multiple QWs.^112,113 In the latest approach in 2015, THz-QCLs have been fabricated via radio-frequency MBE (RF-MBE) and a metal-organic chemical vapor deposition (MOCVD) on MOCVD-growth AlGaN/AlN templates grown on c-plane sapphire substrates. The number of active regions and wave-functions, contributed to lasing, were limited to be two QWs and three subband levels, respectively. As a result, lasing at $\sim 5.5$ and $\sim 7.0\mathrm{THz}$ is achieved, which were the highest reported emissions for THz-QCLs up to date.^86,114,115 In the latter work, the selective injection into the upper lasing level and a wide dynamic range of operating current density are realized in order to achieve a higher operating temperature of the THz-QCL. Toward this aim, indirect injection scheme as shown in Fig. 9(a) is considered as the dynamic range of operating current density is not limited by subband alignment of narrow $E_{32}$. As Fig. 9(b) shows, the highest reported operation temperature for GaAs/AlGaAs is 160 K for a 1.9-THz-QCL and 150 K for a 3.8-THz-QCL, whereas GaN-based QCLs can work at room temperature.^116–118 The schematic of the overall GaN-based THz-QCL device is shown in Fig. 9(c).

Fig. 7

NEP against gate voltage ($T=300\mathrm{K}$) inset: responsivity against gate voltage for $f=0.2\mathrm{THz}$.¹⁰⁵

Fig. 8

Conduction-band profile and squared envelope functions of (a) the $\mathrm{GaAs}/{\mathrm{Al}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{As}$, (b) the $\mathrm{GaN}/{\mathrm{A}}_{l0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}$, and (c) the $\mathrm{ZnO}/{\mathrm{Mg}}_{0.15}{\mathrm{Zn}}_{0.85}\mathrm{O}$ QC gain media considered in this study. (d) Calculated fractional population inversion of the THz-QC structures of (a) (dash-dotted line), (b) (solid line), and (c) (dashed line), as a function of temperature. Reprinted from Ref. 109, with permission of AIP.

AlGaN/GaN QWs are also proposed for absorption of THz radiation.¹¹⁹ Reduction in Al mole fraction causes the QW width to increase, and thus ISB transitions in these QWs are adjustable to be between 1.0 to $10\mu \mathrm{m}$.¹²⁰ For absorption in the THz frequency range, plasma-assisted MBE with tunable absorption from 53 to $160\mu \mathrm{m}$, with respect to doping level and geometrical variations, has been reported.¹²¹ In 2016, researchers from Boston University reported photocurrent peaks near 10 THz for THz-ISB photodetectors, which are developed based on GaN/AlGaN QWs grown on a free-standing semipolar GaN substrate.¹²² Photocurrent spectrum and the conduction-band lineup of the semipolar GaN/AlGaN QW infrared photodetector (QWIP) developed by this research group, as shown in Fig. 10.

Fig. 9

(a) Schematic view of QC structures for resonant tunneling injection scheme (three-level) and indirect injection scheme (four-level). (b) Superiority of GaN-based QCLs in THz regime. (c) Schematic of the overall GaN-based THz-QCL device structure and the device photograph after having mounted to the heat sink. Reprinted from Refs. 86 and 94, with permission of SPIE.

2.5.

GaN-Based Negative Differential Resistance Diode Oscillators

Diodes with NDR have been conventionally used for generation of high-frequency high-power microwave signals. In this regard, GaAs and InP Gunn diodes can be named. Energy-relaxation time in GaAs is around 10 ps. As a result, GaAs exhibits sharp suppression in output power in frequencies higher than 100 GHz and thus cannot be used for the upper THz band applications. Wide-bandgap III to V nitrides exhibit bulk NDR effect in threshold fields above $80\mathrm{kV}/\mathrm{cm}$.¹²³ In addition, thanks to shorter energy relaxation time in GaN among other conventional III to V semiconductor materials,¹²⁴ GaN-based NDR diode oscillators offer much higher electron velocity and reduced time constants compared to conventional GaAs Gunn diodes. As a result, GaN-based NDR diodes can generate frequencies in the THz regime. The NDR relaxation frequency $f_{\mathrm{NDR}}$ of GaAs is reported as $\sim 100\mathrm{GHz}$, whereas it is $\sim 1\mathrm{THz}$ for GaN-based NDR devices for intervalley transfer-based NDR, and $\sim 4\mathrm{THz}$ for of inflection-based NDR. The first GaN-NDR was fabricated through growing GaN layer using metalorganic vapor-phase epitaxy (MOVPE) by a research group at the University of Michigan in 2000.^125,126 The schematic and output power spectrum of THz GaN-based NDR oscillator is shown in Fig. 11. In this device, a $3\text{-}\mu \mathrm{m}$ thick, $n$-type with a doping concentration of $1\times {10}^{17}{\mathrm{cm}}^{-3}$ GaN active layer is sandwiched between anode and cathode, which are both made of GaN with thicknesses and doping concentrations of $0.1\mu \mathrm{m}$ and $1\times {10}^{19}{\mathrm{cm}}^{-3}$, respectively. The diameter of the diode is $50\mu \mathrm{m}$. GaN active layer can be doped significantly higher than that of GaAs. As a result, current levels, and thus output powers, of GaN devices can be up to four times higher than that of GaAs-based devices.

Fig. 10

(a) Conduction-band lineup of the semipolar GaN/AlGaN QWIP developed in this work, under an externally applied voltage bias of about $20\mathrm{mV}/\text{period}$. The squared envelope functions of the bound-state subbands of each well are also shown, referenced to their respective energy levels. (b) Photocurrent spectrum measured at 10 K under an applied voltage of 1.2 V. Reprinted from Ref. 122, with permission of AIP.

2.6.

GaN Hetero-Dimensional Schottky Diode for THz Detection

The first experimental demonstration of THz detection by AlGaN/GaNHDSD for detection of THz radiation has been realized in 2006 by researchers from Rensselaer Polytechnic Institute.¹²⁷ The schematic of this device is shown in Fig. 12. AlGaN/GaN HDSD exhibited reasonable performance in 2.24 THz and higher frequencies. Compared with AlGaAs/InGaAs/GaAs-based THz detectors that were first reported in 1992 by the same research group,¹²⁸ the cutoff frequency of AlGaN/GaN-based HDSD device is tremendously higher. This superiority of AlGaN/GaN-based HDSD has been achieved thanks to the high 2DEG concentration, which is up to 20 times higher than that of the GaAs-based devices. This high 2DEG concentration lowers the series resistance tremendously.

Fig. 11

(a) Schematic of GaN NDR diode oscillator. (b) Simulated output power spectrum of THz-GaN-based NDR oscillator.¹²⁶

2.7.

GaN-Based Impact Avalanche Transit Time Diodes for THz Frequency Range

Monte Carlo simulations of GaN-based IMPATT diodes predict a promising role for these devices in the THz frequency range. The maximum is predicted to occur at 0.45 THz, and the cutoff frequency is on the order of 0.7 THz. The high-frequency operation of GaN-based IMPATT diodes is achievable thanks to superiorities of GaN over GaAs and Si. These superiorities can be named as, high electron drift velocity, high ionization rates, and less pronounced electron relaxation. In addition, thanks to high thermal conductivity of GaN, these devices can operate at high DC current densities.¹²⁹ Moreover, in such devices, fluctuations of the space charges affect the generation of electron–hole pairs as a negative feedback. As a result, the avalanche noise is predicted to be suppressed up to three times of magnitude for the current multiplication factor greater than ten.¹³⁰

2.8.

GaN-Based Planar Gunn Diode for Terahertz Applications

In 2016, researchers from Xidian University proposed a GaN-based planar Gunn diode for THz applications. Utilizing GaN provides high electron concentration of 2DEG which helps the fast formation of the dipole domain layer. Hence, the dead zone length is decreased by reducing the recess layer near the cathode which results in enhancing the RF output power.¹³¹ For reducing the dead zone, donor-like traps near the cathode are removed. As a result, the electron concentration of this region becomes higher than other regions, and thus it acts as a $n^{+}$ doped layer of the Gunn diode. The schematic structure of GaN-HEMT liked planar Gunn is shown in Fig. 13.

Fig. 12

Schematic of (a) lateral and (b) vertical Schottky diode under study. Reprinted from Ref. 127, with permission of IEEE.

2.9.

GaN-Based High-Electron-Mobility Transistors as Terahertz Detectors Based on Self-Mixing

As pioneers, Sun et al.¹³² fabricated a GaN/AlGaN HEMT using optical lithography in 2011. This GaN/AlGaN HEMT can work at room temperature and detect THz radiation via self-mixing with a responsivity of $3.6\mathrm{kV}/\mathrm{W}$. By taking localized THz fields into account, they also developed a quasi-static self-mixing model that describes the detector characteristics such as magnitude and polarity of the photocurrent.¹³³ In 2016, researchers from Massachusetts Institute of Technology and Agency for Science, Technology, and Research in Singapore proposed a new model for GaN HEMTs. This model can explain both polarity and magnitude of the photocurrent. This model helped to design of GaN-based HEMTs with asymmetric pads. Thanks to this model, the detection responsivity is enhanced by one order of magnitude.¹³⁴ The schematics of the cross section of the proposed GaN HEMT detector, antenna structures with traditional symmetric gate, and the proposed asymmetric gate are shown in Fig. 14.

Fig. 13

The schematic structure of GaN-HEMT liked planar Gunn. Reprinted from Ref. 131, with permission of John Wiley and Sons publication.

2.10.

Antenna-Coupled Field-Effect Transistors for the Plasmonic Detection of THz Radiation

In 2016, antenna-coupled field-effect transistors (TeraFETs) for the plasmonic detection of THz radiation were proposed and realized using a $0.25\text{-}\mu \mathrm{m}$ AlGaN/GaN process.^135,136 The fabricated TeraFETs could detect frequencies up to 0.59 THz. This frequency is nearly twice of that of the implemented SiGe:C-based CMOS THz transmitters in 2015 that could emit frequencies up to 0.32 THz.^137,138 TeraFETs have the potential of being realized by CMOS technology and thus provide a promising building block for implementation of compact economical THz cameras. It is observed that such devices show good potential for edge detection and enhanced spatial resolution for THz imaging applications. Similar to asymmetrical GaN-based HEMTs proposed by Wang et al.¹³¹, introduced in Sec. 2.9, Fig. 13, the TeraFETs are asymmetrical. This asymmetry is adopted due to the fact that it has been proven that asymmetric coupling is required for optimal power detection. The detector designs of bow-tie-antenna-coupled TeraFETs and the schematic of the TeraFET are shown in Fig. 15.

Fig. 14

Schematics of the antenna structures with (a) traditional symmetric gate and (b) proposed asymmetric gate. (c) The schematic of cross section of the proposed GaN HEMT detector. Reprinted from Ref. 134, with permission of Elsevier Ltd. publication.

3.1.

THz-Time-Domain Spectroscopy for Characterization of GaN

The earliest comprehensive THz-TDS analysis of semiconductor materials has been reported by researchers from IBM in 1989.^139–141 However, the earliest THz-TDS characterization of the optical properties of GaN epitaxial films has been reported by Bu et al.¹⁴² They observed the far-infrared transmission of the $\mathrm{GaN}/{\mathrm{Al}}_2{\mathrm{O}}_3$ samples, and as a result, they could measure absorption coefficient, mobility, and carrier density in the 1- to 3-THz regime. In one of the early characterizations of GaN in THz frequency regime, which is reported by Zhang et al., the complex conductivity and dielectric function of GaN in the frequency range of 0.1 to 4 THz are reported.¹⁴³ Nagashima et al.^144,145 have reported a comprehensive electrical characterization of GaN thin films using THz TDS. In their work, DC resistivity of the GaN films with various free carrier densities and mobilities of the free carriers in lightly doped GaN films were calculated according to the to the Drude model. It is found that the temperature dependence of the mobilities for the lightly doped films shows a peak at 150 K, and the measured DC resistivity shows good agreement with those obtained by the conventional contact measurements. Tsai et al. reported indexes of refraction, extinction constants, and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz using THz-TDS. They used the Kohlrausch model fit which not only provides the mobility of the free carriers in the GaN film but also estimates the relaxation time distribution function and average relaxation time.¹⁴⁶ In the most recent work, the oscillating dielectric function at various temperatures within THz frequencies is obtained by Fang et al. in 2015. The concentration, electron lifetime, and temperature dependences of the point defects in GaN thin films were obtained. They reported that the concentration of the point defects decreases with the rise of temperature while the electron lifetime shows positive temperature dependence.¹⁴⁷

3.2.

Laser-Induced THz Emission Spectroscopy for Visualization of GaN Defect Density and Surface Potential

Laser-induced THz emission spectroscopy (LTEM) has been used as an alternative to THz-TDS for characterization of optical and electrical properties of semiconductor materials.¹⁴⁸ LTEM has been also used for inspection of semiconductor devices.^149–151 Sakai et al.¹⁵² have demonstrated laser-induced THz emission from the surface of GaN, which can be used for determination of the defect density and surface potential. They observed that when the GaN surface is excited by ultraviolet femtosecond laser pulses, the THz emission is enhanced by defects related to yellow luminescence (YL). As shown in Fig. 16, this phenomenon is explained by band bending as a result of trapped electrons at defect sites. The result of this visualization and its consistency with YL is shown in Fig. 16. The importance of this characterization technique is due to its applications for evaluation of the distribution of the nonradiative defects, which are undetectable with photoluminescence. Avoiding these defects leads to the realization of normally off GaN devices, which have an important part in energy-efficient power devices.

Fig. 15

Three detector designs (a)–(c) of bow-tie-antenna-coupled TeraFETs. The graph at the bottom displays a cross section of the respective AlGaN/GaN device. (a)–(c) Enlarged views of the gate regions illustrate the charge density waves propagating from the source side (red) and drain side (blue) of the gate for homogeneous illumination. Arrows indicate magnitude and direction of rectification responses of both waves. Reprinted from Ref. 136, with permission of IEEE.

3.3.

THz Electro-Modulation Spectroscopy of GaN Electron Transport

Engelbrecht et al.¹⁵³ have reported THz electromodulation spectroscopy for studying charge transport in $n$-type GaN. TDS significantly reduces mobility, and thus THz electromodulation spectroscopy is proposed as a promising tool for characterization of low doping densities where classical techniques are not efficient. As a result, the accessible frequency range extends from about 0.2 to 2.8 THz. The experiment setup, which is shown in Fig. 17 includes GaN grown on sapphire by MOVPE. Schottky devices were fabricated for satisfying the requirement for switching of the electron sheet density $n_{2\text{-}\mathrm{D}}$ within the semiconductor. This switching of $n_{2\text{-}\mathrm{D}}$ is required for realizing THz electromodulation spectroscopy. In the mentioned work, the conductivity effective mass of the electrons and the relaxation times have been computed.

Fig. 16

(a) The LTEM image of $n$-type GaN with an excitation wavelength of 260 nm. (b) The THz waveforms at two points in (a). Line colors correspond to point colors in (a). (c) The PL spectra of GaN at two characteristic points shown in (d) as blue and orange points. The excitation wavelength is 345 nm. (d) PL 2-D mapping using the intensity of YL peak with the excitation wavelength of 345 nm.¹⁵²

Fig. 17

Schematic band diagram of the devices in equilibrium. The sheet density of the electrons is modulated by the bias applied to the Schottky contact. The modulation affects absorption and dispersion of the THz pulses transmitted through the device. Reprinted from Ref. 153, with permission of AIP.

In addition to the mentioned properties, wide bandgap semiconductor materials and particularly GaN exhibit very interesting properties that can be utilized for implementing novel applications. In this respect, the unexpected effect of massive charge neutralization by enhanced exciton formation at very high plasma densities in wide-gap materials such as GaN has been demonstrated.¹⁵⁴ In another report, ultrafast internal field dynamics with lattice dynamics in GaN-based QWs are correlated and the measured THz strain amplitudes have been among the largest observed ultrafast photoacoustic generation.¹⁵⁵