THz Oscillations in a GaN Based Planar Nano-Device

doi:10.4236/jcc.2013.17012

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Journal of Computer and Communications, 2013, 1, 50-53

Published Online December 2013 (http://www.scirp.org/journal/jcc)

http://dx.doi.org/10.4236/jcc.2013.17012

Open Access JCC

THz Oscillations in a GaN Based Planar Nano-Device

K. Y. Xu1, Y. N. Wang2, Z. N. Wang2, J. W. Xiong2, G. Wang2

1Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal

University, Guangzhou, China; 2State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University,

Guangzhou, China.

Email: [email protected]

Received September 2013

ABSTRACT

Gunn oscillations in a GaN based planar nano-device have been studied by ensemble Monte Carlo (EMC) method. Si-

mulation results show that when the channel length of the device reduces to 450 nm, THz oscillations (about 0.3 THz)

can be obtained. Also the phase of the oscillations can be controlled by the initial conditions that excite the Gunn do-

mains. Moreover, through adjusting the phase difference between the oscillations in a double-channels device, which

attained by parallel connecting two single-channel devices, the frequency of the device shifts from 0.3 THz to 0.6 THz.

This phenomenon remains in devices with shorter channel-length, unless the channel-length is too short to support

Gunn osci llations . T he possible underlying mec ha nisms are a lso discuss ed.

Keywords: THz Oscillation; Gunn; Monte Carlo; GaN; Planar Nanodevice

1. Introduction

The terahertz (1 THz = 1000 GHz) electromagnetic wave

has attracted w ide attentions as it could enable very broad

applications ranging from non-destructive imaging and

spectroscopy of biological materials, remote detection of

hidden objects and explosives, to manipulations of quan-

tum states in semiconductors [1,2]. Moreover, develop-

ment of semiconductor THz electronic devices is also cer-

tainly paramount and timely to future generation of large-

volume information processing and high-performance

computations. However, the development of THz tech-

nology is so far largely hampered by the lack of reliable,

solid-state sources operating at room-temperature [3].

One possible way of developing THz devices is to

adapt well-known mechanisms that have been already

utilized in microwave f ield for higher frequency applica-

tions. Among them, Gunn Effect in GaN is considered as

one of the most promising candidates for further THz

sources, which has attracted wide attention recently [4-8].

Heat dissipation is considered as one of the most chal-

lenging issues for practical GaN-based Gunn sour ces. To

conquer this problem, planar devices, such as SSDs [9],

may be a suitable choice, since planar architecture allows

easier design for heat dissipation [5,8]. In addition, in

planar devices the electrodes are connected side by side

to the active semiconductor layer rather than placed on

top of each other, as in conventional multilayered vertical-

structured devices, resulting in very low parasitic capa-

citances. As such, high speeds are attainable.

In this work, we focus on studying the behaviors of

Gunn oscillations in GaN based SSDs at room tempera-

ture. SSDs as shown in Figure 1 are planar nanodevices,

which not only have high operation speed, up to THz at

room temperature, but also benefit for the propagation of

Gunn domains [10]. The paper is structured as follows.

In Section 2, device structures and simulation models are

introduced. In Section 3, the time-dependent Gunn oscil-

lations are studied under different bias conditions. Then

the relationship between the initial phase of the oscilla-

tion and the exciting condition is established. And moreo -

ver the interaction of Gunn oscillations in adjacent double

channels is further studied under different phase condi-

tions. In Section 4, simulation results are further dis-

cussed.

2. Device Structures and EMC Model

Figure 1(a) shows schematically the top view of a

double-channels device including two SSDs and will be

mentioned as D-SSD in follows. The device is based on a

GaN/AlGaN heterostructure, where a 2DEG is formed at

the hetero-interface with a carrier concentration of 8.0 ×

1012 cm−2 [8]. For a single-channel SSD (S-SSD) framed

by green dashed line in Figure 1(a), the two L-shaped

insulating trenches are etched through the 2DEG layer,

which ensures that electrons have to pass the narrow

channel between the two trenches in order to conduct a

THz Oscillations in a GaN Based Planar Nano-Device

Open Access JCC

(a)

0.5μm0.45μm 1.0μm

Left

terminal 2

Right

terminal

50nm

30nm

0.2μm

30nm

2DEG

Air

1μm

GaN

AlGaN

Etched area

50nm

Left

terminal 1

30nm

0.5μm

(b)

Figure 1. Schematic top view (a) and side view (b) of the

simulated T-SSD (not to scale). The gray areas and the

white areas in the top view represent 2DEG and insulating

trenches, respectively. The green dash-lines delineate the

typical structure of single SSDs. An interface of AlGaN/

GaN heterostructures in (b) is just 30 nm bellows the de vice

surface, at which a sheet of 2DEG forms.

current between the left and right terminals. The two

S-SSDs are both designed with a channel width of 50 nm

and a trench width of 30 nm. Other geometric parameters

are defined in Figure 1(a). In the D-SSD, the two S-

SSDs have separated left terminals, but share the same

right terminal. The design of sharing the same right ter-

minal will benefit the interactions between channels, and

that of using different left terminals will entitle us to con-

trol the Gunn oscillations through applied voltage s .

In order to obtain the operation properties of the de-

vices, a semi-classical 2D EMC method self-consistently

coupled with 3D Poisson equations is used here. This

2D-3D combined model is developed from our entirely

2D EMC model, which has been used in earlier work

[10-13]. Details of the above 2D-3D combined model

and the comparison with fully 3D EMC method [14,15],

can be found in our recent work [16]. Figure 1(b) shows

schematically the side view of the simulated SSDs. In

order to properly include the 3D electric-field coupling,

Poisson equations are solved in a domain beyond the

geometric structure of the SSDs. As one can find from

Figure 1(b), a volume with a height of one micrometer

above the device surface is also included. The dielectric

constant used in the simulations for Air, AlGaN and GaN

are 1, 8.5 and 8.9, respectively. In order to model the

influence of surface states at the semiconductor-air inter-

face, a uniform negative charge density, 0.8 × 1012 cm−2,

is also added at the edge of the insulating trenches during

the simulations. All simulations are carried out at room

temperature.

3. Simulation Results

In this section, EMC method is used to study the Gunn

oscillations in S-SSDs and D-SSDs. Sim ulations are firstly

carried out on S-SSDs under different bias condition to

study the voltage dependence and the initial phase of

Gunn oscillations. Then the interaction of Gunn oscilla-

tions is further studied in D-SSDs under different phase

conditions.

3.1. Single-Channel Devices

For S-SSD simulations, the left terminal of the device is

always grounded and the current output from the right

terminal is recorded. Fig ure 2 shows the time-dependent

current output of an S-SSD when a serial of step voltages

are applied on the right terminal. A current peak with

sharp rise and fall is shown after each abrupt change of

the applied voltage, which is anticipated as a result of the

charging of parasitic capacitances in the device [10]. This

parasitic-capacitance-induced current-peak will be men-

tioned as PCIC peak in the following studies. The current

after each PCIC peak increases with the applied voltage

and shows obvious oscillation behaviors when the ap-

plied voltage beyond 16V. Since the length of the chan-

nel is 450 nm, the threshold electric field for Gunn os cil-

lation would be about 0.4 MV/cm, which agrees with

recent experimental results [4]. Recently, by using an

entirely 2D EMC model, SSDs with a wider channel-

width have been studied and a special mode with two

domains simultaneously forming in the channel have

been found, but the mechanism is still puzzled [8]. The

period of the oscillations shown in Figure 2 is about 3 ps,

corresponding to frequency about 0.3 THz. The period of

the oscillations is also the time for Gunn domain to travel

through the channel, so that the drift velocity of the Gunn

domain should be about 1.5 × 107 cm/s, which is close to

the saturated velocity of the electrons in GaN [17]. This

result reveals that in our case, like those in traditional

Gunn diodes, only one effective domain simultaneously

exists.

Figure 2. Current responses of a single SSD showed in Fig-

ure 1 under the application of a series voltage steps.

THz Oscillations in a GaN Based Planar Nano-Device

Open Access JCC

3.2. Phase Control

Further investigations tell us that the initial phase of the

Gunn oscillations is dependent on what time the step

voltage is applied. If the time interval of two step voltag -

es is Ti, the phase-shift between the corresponding ex-

cited-oscillations will be 2πTi/T, where T is the period of

the oscillations. Typical results are shown in Figure 3.

The applied step voltages change their values from 0 V

to 21 V at 2 ps and 3.5 ps respectively, resulting in two

PCIC peaks. Since these PCIC peaks are too strong and

with non-needing information for our topic, only parts of

them are shown in Figure 3. One can find that the phase

of the two oscillations is opposite, because the time in-

terval is just half of the oscillation period.

3.3. Double-Channel Devices

During D-SSD simulations, the right ter minal is grounded

and two different negative step voltages are applied to

the two left terminals, respectively. Results for special

time interval Ti = 0 ps and Ti = 1.5 ps are shown in Fig-

ure 4. PCIC peaks are also not shown in whole as those

in Figure 3. The time-dependent output-current induced

by step voltages with zero time-interval is similar to that

in S-SSD. However, that induced by step voltages with

1.5 ps interval shows obvious second-harmonic oscilla-

tion with period of about 1.5 ps, just half of that in S-

SSD, corresponding to a frequency of about 0.6 THz.

The distinct deviation of current output between in

phase and out phase oscillations may come from the fol-

lowing facts. The two channels are identified, so the po-

tential distributions along the two channels should be the

same for the in-phase case. But for the out-phase case,

the potential distributions must be different and might

also changes with time. As a result, in the out -phase case,

the two oscillations will mutually modulate. For simpli-

fication, the Gunn oscillations can be assumed to obey

sinusoidal function with frequency of f, so that the mu-

tual modulation could be reasonably described with the

same sinusoidal function. Consequently, the current out-

put would be proportional to the square of the sinusoidal

function, which possesses a fr e q uency of 2f .

Figure 3. Current responses of a single SSD showed in Fig-

ure 1 when the applied voltage changes from 0 to 21 V at (a)

2 ps and (b) 3.5 ps.

Figure 4. Current re sponses of a T-SSD showed in Figure 1

for (a) without and (b) with self-gating effect.

It is well known that when two oscillators are put to-

gether, they would show synchronized oscillations [18].

As such, more simulations are carried out for D-SSD in

the out-phase case to exa mine the persis tence of the above

modulations. Results show that even in a much longer

simulation time (longer than 300 ps which including at

least 600 periods of oscillations), the waveform of the

current output does not change. Moreover, further inves-

tigations show that the above phenomenon still exists in

shorter-channel D-SSDs so that oscillations with higher

frequency can be obtained. For an instance, when the

channel length is reduced to 300 nm, the corresponding

frequency is raised to about 1 THz, results not shown

here.

4. Conclusions

In conclusions, Gunn oscillations in GaN based S-SSDs

and D-SSDs have been studied by ensemble Monte Carlo

(EMC) method in detail. We show that since the channel

length of the device is extremely short (only 450 nm) the

oscillations in S-SSDs operate with a frequency about 0.3

THz. Also the initial phase of Gunn oscillations can be

controlled by applied voltage. Moreover, for D-SSD s, by

adjusting the phases of two oscillations in each channel

to be contrary, the frequency of Gunn oscillation can be

increased from 0.3 THz to 0.6 THz. This phenomenon

may attribute to the mutual modulations between the two

oscillations and occurs even in shorter channels so long

as Gunn oscillation can occur, resulting in oscillations

with frequency up to 1 THz.

5. Acknowledgements

This work was supported by FOK YING TONG Educa-

tion Foundation (No. 122004), Natural Science Founda-

tion of Guangdong Province, China (No. S2013010012711)

and NSFC (Grands U0934002).

REFERENCES

[1] M. Sherwin, “Terahertz Power,” Nature, Vol. 420, 2002,

pp. 131-133. http://dx.doi.org/10.1038/420131a

[2] M. Tonouchi, “Cutting-Edge Terahertz Technology,”

THz Oscillations in a GaN Based Planar Nano-Device

Open Access JCC

Nature Photonics, Vol. 1, 2007, pp. 97-105.

http://dx.doi.org/10.1038/nphoton.2007.3

[3] P. H. Siegel, “Terahertz Technology,” IEEE Transactions

on Microwave Theory Technology, Vol. 50, No. 3, 2002,

pp. 910-928. http://dx.doi.org/10.1109/22.989974

[4] N. Ma, B. Shen, F. J. Xu, L. W. Lu, Z. H. Feng, Z. G.

Zhang, S. B. Dun, C. P. Wen, J. Y. Wang, F. Lin, D. T.

Zhang and M. Sun, “Current-Controlled Negative Diffe-

rential Resistance Effect Induced by Gunn-Type Instabil-

ity in n-Type GaN Epilayers,” Applied Physical Letters,

Vol. 96, 2010, Article ID: 242104.

http://dx.doi.org/10.1063/1.3455070

[5] A. Íñiguez-de-la-Torre, I. Íñiguez-de-la-Torre, J. Mateos

and T. González, “Correlation between Low-Frequency

Current-Noise Enhancement and High-Frequency Oscilla-

tions in GaN-Based Planar Nanodiodes: A Monte Carlo

Study,” Applied Physical Letters, Vol. 99, 2011, Article

ID: 062109. http://dx.doi.org/10.1063/1.3613956

[6] Y. Hao, J. F. Zhang, B. Shen and X. Y. Liu, “Progress in

Group III Nitride Semiconductor Electronic Devices,” J.

Semicond., Vol. 33, No. 8, 2012, Article ID: 081001.

http://dx.doi.org/10.1088/1674-4926/33/8/081001

[7] L. A. Yang, S. Long, X. Guo and Y. Hao, “A Compara-

tive Investigation on Sub-Micrometer InN and GaN Gunn

Diodes Working at Terahertz Frequency,” Journal of Ap-

plied Physics, Vol. 111, 2012, Article ID: 104514.

http://dx.doi.org/10.1063/1.4721667

[8] A. Íñiguez-de-la-Torre, I. Íñiguez-de-la-Torre, J. Mateos,

T. González, P. Sangaré, M. Faucher, B. Grimbert, V.

Brandli, G. Ducournau and C. Gaquière, “Searching for

THz Gunn Oscillations in GaN Planar Nanodiodes,”

Journal of Applied Physics, Vol. 111, 2012, Article ID:

113705. http://dx.doi.org/10.1063/1.4724350

[9] A. M. Song, M. Missous, P. Omli ng, A. R. Peaker, L, Sa -

muelson and W. Seifert, “Unidirectional Electron Flow in

a Nanometer-Scale Semiconductor Channel: A Sel f-Swit-

ching Device,” Applied Physical Letters, Vol. 83, 2003, p.

1881. http://dx.doi.org/10.1063/1.1606881

[10] K. Y. Xu, G. Wang and A. M. Song, “Gunn Oscillations

in a Self-Switching Nanodiode,” Applied Physical Letters,

Vol. 93, 2008, Article ID: 233506.

http://dx.doi.org/10.1063/1.3042268

[11] K. Y. Xu, X. F. Lu, G. Wang and A. M. Song, “Strong

Spatial Dependence of Electron Velocity, Density, and

Inter-Valley Scattering in an Asymmetric Nanodevice in

the Nonlinear Transport Regime,” IEEE Transactions on

Nanotechnology, Vol. 7, No. 4, 2008, pp. 451-457.

http://dx.doi.org/10.1109/TNANO.2008.926348

[12] K. Y. Xu, X. F. Lu, G. Wang and A. M. Song, “Enhanced

Terahertz Detection by Localized Surface Plasma Oscil-

lations in a nanoscale unipolar diode,” Journal of Applied

Physics, Vol. 103, 2008, Article ID: 113708.

http://dx.doi.org/10.1063/1.2937175

[13] K. Y. Xu, X. F. Lu, A. M. Song and G. Wang, “Terahertz

Harmonic Generation Using a Planar Nanoscale Unipolar

Diode at Zero Bias,” Applied Physical Letters, Vol. 92,

2008, Article ID: 163503.

http://dx.doi.org/10.1063/1.2907490

[14] T. Sadi, F. Dessenne and J.-L. Thobel, “Three-Dimen-

sional Monte Carlo Study of Three-Terminal Junctions

Based on InGaAs/InAlAs Heterostructures,” Journal of

Applied Physics, Vol. 105, 2009, Article ID: 053707.

http://dx.doi.org/10.1063/1.3087703

[15] T. Sadi and J.-L. Thobel, “Analysis of the High-Fre-

quency Performance of InGaAs/InAlAs Nanojunctions

Using a Three-Dimensional Monte Carlo Simulator,”

Journal of Applied Physics, Vol. 106, 2009, Article ID:

083709. http://dx.doi.org/10.1063/1.3248358

[16] K. Y. Xu, J. W. Xiong, A. M. Song and G. Wang, “Ef-

fects of Three-Dimensional Electric-Field Coupling on a

Side-Gated Nanotransistor,” Semicond. Sci. Technol., Vol.

26, No. 9, 2011, Article ID: 095026.

http://dx.doi.org/10.1088/0268-1242/26/9/095026

[17] S. Chen and W. Gang, “High-Field Properties of Carrier

Transport in Bulk Wurtzite GaN: A Monte Carlo Pers-

pective,” Journal of Applied Physics, Vol. 103, 2008,

Article ID: 023703. http://dx.doi.org/10.1063/1.2828003

[18] B. Razavi, “Mutual Injection Pulling between Oscillators,”

IEEE 2006 Custom Int egrated Circuits Conference (CICC),

California, 10-13 September 2006, pp. 675-678.