High performances III-Nitride Quantum Dot infrared photodetector operating at room temperature

High performances III-Nitride Quantum Dot

infrared photodetector operating at room

temperature

A. Asgari1,2,*

and S. Razi,1

1Photonics-Electronics Group, Research Institute for Applied Physics, University of Tabriz, Tabriz 51665-163, Iran 2 School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Crawley, WA

6009, Australia

*[email protected]

Abstract: In this paper we present a novel long wave length infrared

quantum dot photodetector. A cubic shaped 6nm GaN quantum dot (QD)

within a large 18 nm 0.2 0.8

Al Ga N QD (capping layer) embedded in

0.8 0.2Al Ga N has been considered as the unit cell of the active layer of the

device. Single band effective mass approximation has been applied in order

to calculate the QD electronic structure. The temperature dependent

behavior of the responsivity and dark current were presented and discussed

for different applied electric fields. The capping layer has been proposed to

improve upon the dark current of the detector. The proposed device has

demonstrated exceptionally low dark current, therefore low noise, and high

detectivity. Excellent specific detectivity (D*) up to ~3 × 108 CmHz

1/ 2/W is

achieved at room temperature.

©2010 Optical Society of America

OCIS codes: (250.250) Optoelectronics; (250.0040) Detectors.

References and links

1. B. F. Levine, “Quantum well infrared photodetectors,” J. Appl. Phys. 74(8), R1–R81 (1993).

2. A. Goldberg, S. Kennerly, J. Little, T. Shafer, C. Mears, H. Schaake, M. Winn, M. Taylor, and P. Uppal,

“Comparison of HgCdTe and quantum-well infrared photodetector dual-band focal plane arrays,” Opt. Eng.

42(1), 30–46 (2003).

3. A. Rogalski, “Infrared Detectors”, NewYork: Gordon and Breach, 155–650 (2000).

4. S. Y. Wang, S. D. Lin, H. W. Wu, and C. P. Lee, “Low dark current quantum-dot infrared photodetectors with

an AlGaAs current blocking layer,” Appl. Phys. Lett. 78(8), 1023 (2001).

5. X. Lu, J. Vaillancourt, and M. J. Meisner, “Temperature-dependent photoresponsivity and high-temperature (190

K) operation of a quantum dot infrared photodetector,” Appl. Phys. Lett. 91(5), 051115 (2007).

6. J. Phillips, P. Bhattacharya, S. W. Kennerly, D. W. Beekman, and M. Dutta. “Self-assembled InAs-GaAs

quantum-dot intersubband detectors,” IEEE J. Quantum Electron. 35(6), 936–943 (1999).

7. S. Y. Wang, M. C. Lo, H. Y. Hsiao, H. S. Ling, and C. P. Lee, “Temperature dependent responsivity of quantum

dot Infrared photodetectors,” Infra. Phys. Technol. 50, 166 (2007).

8. D. Pan, E. Towe, and S. Kennerly, “Normal-incidence intersubband (In, Ga)As/GaAs quantum dot infrared

photodetectors,” Appl. Phys. Lett. 73(14), 1937 (1998).

9. A. Stiff, S. Krishna, P. Bhattacharya, and S. Kennerly, “High-detectivity, normal-incidence, mid-infrared

(λ~4 µm)InAs/GaAs quantum-dot detector operating at 150 K,” Appl. Phys. Lett. 79(3), 421 (2001).

10. L. Jiang, S. S. Li, N. Yeh, J. Chyi, C. E. Ross, and K. S. Jones, “In0.6Ga0.4As/GaAs quantum-dot infrared

photodetector with operating temperature up to 260 K,” Appl. Phys. Lett. 82(12), 1986 (2003).

11. U. Bockelmann, and G. Bastard, “Phonon scattering and energy relaxation in two-, one-, and zero-dimensional

electron gases,” Phys. Rev. B 42(14), 8947–8951 (1990).

12. Z. Ye, J. C. Campbell, Z. Chen, E.-T. Kim, and A. Madhukar, “Noise and photoconductive gain in InAs

quantum-dot Infrared photodetectors,” Appl. Phys. Lett. 83(6), 1234 (2003).

13. S. Chakrabarti, A. D. Stiff-Roberts, P. Bhattacharya, S. Gunapala, S. Bandara, S. B. Rafol, and S. W. Kennerly,

“High-temperature operation of InAs-GaAs quantum-dot infrared photodetectors with large responsivity and

detectivity,” IEEE Photon. Technol. Lett. 16(5), 1361–1363 (2004).

#127291 - $15.00 USD Received 20 Apr 2010; revised 27 May 2010; accepted 15 Jun 2010; published 23 Jun 2010(C) 2010 OSA 5 July 2010 / Vol. 18, No. 14 / OPTICS EXPRESS 14604

14. S. Tang, C. Chiang, P. Weng, Y. Gau, J. Luo, S. Yang, C. Shih, S. Lin, and S. Lee, “High-temperature operation

normal incident 256/spl times/256 InAs-GaAs quantum-dot infrared photodetector focal plane array,” IEEE

Photon. Technol. Lett. 18(8), 986–988 (2006).

15. S. Y. Lin, Y. R. Tsai, and S. C. Lee, “High-performance InAs/GaAs quantum-dot infrared photodetectors with a

single-sided Al0.3Ga0.7 blocking layer,” Appl. Phys. Lett. 78(18), 2784–2786 (2001).

16. H. Lim, W. Zhang, S. Tsao, T. Sills, J. Szafraniec, K. Mi, B. Movaghar, and M. Razeghi, “‘’Quantum dot

infrared photodetectors: Comparison of experiment and theory,” Phys. Rev. B 72(8), 085332 (2005).

17. A. D. Stiff, S. Krishna, P. Bhattacharya, and S. Kennerly, “Normal-incidence, high-temperature, mid-infrared,

InAs-GaAs vertical quantum-dot infrared photodetector,” IEEE J. Quantum Electron. 37(11), 1412–1419 (2001).

18. T. Wang, J. Bai, and S. Sakai, “Influence of InGaN/GaN quantum-well structure on the performance of light-

emitting diodes and laser diodes grown on sapphire substrates,” J. Cryst. Growth 224(1-2), 5–10 (2001).

19. L.-W. Ji, T.-H. Fang, and T.-H. Meen, “Effects of strain on the characteristics of InGaN-GaN multiple quantum

dot blue light emitting diodes,” Phys. Lett. A 355(2), 118–121 (2006).

20. S. Shishech, A. Asgari, R. Kheradmand, “The effect of temperature on the recombination rate of AlGaN/GaN

light emitting diodes,” Opt. Quantum Electron., under press (2010).

21. L. W. Wang, A. J. Williamson, A. Zunger, H. Jiang, and J. Singh, “Comparison of the k⋅p and direct

diagonalization approaches to the electronic structure of InAs/GaAs quantum dots,” Appl. Phys. Lett. 76(3),

339–342 (2000).

22. C. Y. Ngo, S. F. Yoon, W. J. Fan, and S. C. Chua, “Effects of size and shape on electronic states of quantum

dots,” Phys. Rev. B 74(24), 245331 (2006).

23. M. Roy, and P. A. Makasym, “Efficient method for calculating electronic states in self-assembled quantum

dots,” Phys. Rev. B , 68235308 (2003).

24. M. Califano, and P. Harrison, “Presentation and experimental validation of a single-band, constant-potential

model for self-assembled InAs/GaAs quantum dots,” Phys. Rev. B 61(16), 10959–10965 (2000).

25. O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, W. J. Schaff, L. F. Eastman, R.

Dimitrov, L. Wittmer, M. Stutzmann, W. Rieger, and J. Hilsenbeck, “Two-dimensional electron gases induced

by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures,” J. Appl.

Phys. 85(6), 3222 (1999).

26. S. De Rinaldis, I. D’Amico, E. Biolatti, R. Rinaldi, R. Cingolani, and F. Rossi, “Intrinsic exciton-exciton

coupling in GaN-based quantum dots: Application to solid-state quantum computing,” Phys. Rev. B 65(8),

081309 (2002).

27. R. Cingolani, A. Botchkarev, H. Tang, H. Morkoç, G. Traetta, G. Coli, M. Lomascolo, A. Di Carlo, F. Della

Sala, and P. Lugli, H. M. G. Traetta, G. Coli, M. L. A. D. Carlo, F. D. Sala, and P. Lugli, “Spontaneous

polarization and piezoelectric field in GaN/Al0.15Ga0.85N quantum wells: Impact on the optical spectra,” Phys.

Rev. B 61(4), 2711–2715 (2000).

28. M. A. Cusack, P. R. Briddon, and M. Jaros, “Electronic structure of InAs/GaAs self-assembled quantum dots,”

Phys. Rev. B 54(4), R2300–R2303 (1996).

29. A. Asgari, M. Kalafi, and L. Faraone, “The effects of partially occupied sub-bands on two-dimensional electron

mobility in AlxGa1-xN/GaN heterostructures,” J. Appl. Phys. 95(3), 1185 (2004).

30. M. Razeghi, H. Lim, S. Tsao, J. Szafraniec, W. Zhang, K. Mi, and B. Movaghar, “Transport and photodetection

in self-assembled semiconductor quantum dots,” Nanotechnology 16(2), 219–229 (2005).

31. Z. Ye, J. C. Campbell, Z. Chen, E.-T. Kim, and A. Madhukar, “Normal-Incidence InAs Self-Assembled

Quantum-Dot Infrared Photodetectors With a High Detectivity,” IEEE J. Quantum Electron. 38, 1534–1538

(2002).

32. X. Lu, J. Vaillancourt, M. J. Meisner, and A. Stintz, “Long wave infrared InAs-InGaAs quantum-dot infrared

photodetector with high operating temperature over 170 K,” J. Phys. D Appl. Phys. 40(19), 5878–5882 (2007).

33. P. Bhattacharya, X. H. Su, S. Chakrabarti, G. Ariyawansa, and A. G. U. Perera, “Characteristics of a tunneling

quantum-dot infrared photodetector operating at room temperature,” Appl. Phys. Lett. 86(19), 191106 (2005).

34. A. G. U. Perera, P. V. V. Jayaweera, G. Ariyawansa, S. G. Matsik, K. Tennakone, M. Buchanan, H. C. Liu, X.

H. Su, and P. Bhattacharya, “Room temperature nano- and microstructure photon detectors,” Microelectron. J.

40(3), 507–511 (2009).

35. H. R. Saghai, N. Sadoogi, A. Rostami, and H. Baghban, “Ultra-high detectivity room temperature THZ-IR

photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD),” Opt. Commun.

282(17), 3499–3508 (2009).

1. Introduction

The most advanced III-V mid and long wavelength infrared (MWIR and LWIR) detectors, to

date, is the quantum well infrared photodetectors (QWIPs) which utilize intersubband or

subband to continuum transitions in quantum wells [1,2]. QWIPs have demonstrated excellent

imagery performance and also extremely uniformity across a large area, which increases the

pixel operability in a focal plane array without the reliance on correction algorithms needed

for MCT detectors. However, QWIPs require lower operating temperature, owing to their


higher thermionic emission rates. The operating temperature for QWIPs is lower than for

MCT detectors, because the thermionic emission in MCT, for equivalent device parameters,

is approximately five orders of magnitude less than in a QWIP [3]. Another serious drawback

is the fact that, the n-type QWIPs cannot detect normal incidence radiation, due to the

polarization selection rules [1]. Consequently, QWIPs require the addition of light couplers,

such as surface gratings, which add to the cost and complexity.

Recently, quantum dot infrared photodetectors (QDIPs) have been emerged as a potential

alternative to MCT and QWIPs [4–7]. The advantages of QDIPs, can mainly categorize in

three parts, (i) The three dimensional quantum confinement of the carriers, which results in

the δ -like density of states, and sensitivity to the normal incident radiation, without the use

of a grating or corrugations, as is often done in QWIPs [8–10], (ii) reduced electron-phonon

scattering, so long excited state lifetime, and high current gain [7, 11, 12]. (iii) The QDIP

technology is believed to be promising for high-temperature operations [13, 14].

In order to improve the performance of these detectors, different structures and materials

have been investigated [4–8, 15, 16]. It has been shown that a current blocking layer can be

effectively used to reduce the dark current. Lin et al. [15] and Stiff et al. [9, 17] have reported

QDIPs with a single AlGaAs blocking layer on one side of the InAs/GaAs QD layers. Wang

et al. [4] introduced a thin AlGaAs barrier layer between the InAs QDs. This layer filled the

area between the dots but left the top of the dots uncovered. An improvement in the

detectivity relative to similar devices without this barrier layer has been deduced in these

papers. On the other hand, in the last few years, the III-nitride QDs have been extensively

studied for their potential use in transistors, lasers and light emitting diodes [18–20]. GaN and

its alloys with AlN have strange properties such as larger saturation velocity, wide band gap

and higher thermal stability, in comparison to the usual and prevalent III-V materials. But

unfortunately, they still suffer from a certain lack of knowledge in terms of fundamental

material parameters, and they are in their early stage. Here we tried to investigate special kind

of QDs with these materials. The Eigen functions, Eigen values, oscillator strength and other

physical parameters calculated in the first stage. Then, the detector parameters such as

responsivity and dark current were evaluated precisely, by considering their temperature

dependence. Specific detectivity used as figure of merit, and its peak was calculated at

function of temperatures for different applied bias.

2. Model derivation

To model the device, a cubic shaped 6nm GaN QD within a large 18 nm 0.2 0.8

Al Ga N QD

embedded in 0.8 0.2

Al Ga N layer is assumed. The proposed structure has been shown in Fig. 1.


Fig. 1. The proposed cubic shaped GaN QD within a large 0.2 0.8

Al Ga N QD.

Five layers with QD density of 24 310Nd m−= are used as active region of the device. We

have assumed the large QDs are very close to each other.

In order to study the electronic structures, different methods have been experienced

[21–24]. The single band method is used in this study. In the frame work of the envelope

function, and the effective mass theory, the Hamiltonian can be written as [22]:

2

*

1( , , ).

2 ( , , )H v x y

m x y

−= ∇ ∇ +ℏ

zz

(1)

In which *m is the electron effective mass and is given by:

( )0.2 0.8

0.8 0.2

*

*

*

, , ,

GaN

Al Ga N

Al Ga N

m in QD

m x y m in capping layer

m in barrier

∗

=

z

(2)

and

( )0

, , .c

inside GaN QDV x y

E else

=

∆z (3)

Wherec

E∆ is the conduction and valance bands discontinuity [25]:

0.7( 6.13 (1 ) 3.42 (1 ) ) ,0

E x x x x E eVc g∆ = × + − × − − − (4)

where x notifies Al molar fraction and is considered 0.2 in our calculations for capping layer

and 0.8 for the barrier.

As the system needs an applied electric field to operate and also has a strong built in

electric field, one has to take into account the total fields effect in the Hamiltonian:

2

*

1( , , ) .

2 ( , , )H v x y e F r

m x y

→ →−= ∇ ∇+ +ℏ

zz

(5)

Where F→

denotes both the external and built in electric fields. It should be mentioned that

III- nitrides in the wurtzite phase have a strong spontaneous and piezoelectric polarization.


The abrupt variation of the polarization at the interfaces gives rise to large polarization sheet

charges which creates the built-in electric field. Therefore, the optical properties of wurtzite

AlGaN/GaN QDs are affected by the 3D confinement electrons and the strong built-in electric

field. This causes the simulation of the systems extremely challenging task.

The Built in electric field which applied in the equations is [26]:

0

( ).

( )

br d

br tot tot

d

d br br d

L P PF

L Lε ε ε−

=+

(6)

Where ( )br d

ε is the relative dielectric constant of the barrier (dot), /br d

totP is the total

polarization and /br d

L is the width of the barrier and height of the dot.

/ / /

.br d br d br d

tot pie o spP P P= +

z (7)

The Piezoelectric polarization includes: one part induced by the lattice mismatch (ms), and

the other caused by thermal strain (ts): / / /br d br d br d

pie o ms tsP P P= +

z, where

/ 13 0

31 33

33

2( )( )br d

ms

c a aP e e

c a

−= − and 4 23.2 10 /d

tsP c m−= − × [26].

31e and

33e are the

piezoelectric coefficients, 13

c and 33

c are elastic constants, and ‘a’ is the lattice constant of

1x xAl Ga N− and is

o

(0.077 3.189) Aa x= + . (All other material parameters can be found in

[27]).

The spontaneous polarization for 1x x

Al Ga N− is Al molar fraction dependent and is given

by: ( 0.052 0.029)sp

P x= − − .

To solve the Schrödinger equation, assuming that the wave functions are expanded in

terms of the normalized plane waves [22]:

( ) ( ), , , ,

, ,

1, , exp . . . .nx ny n nx ny n nx ny nz

nx ny nx y

x y a i k x k y kL L L

ψ = + +∑z z

zz

z z (8)

wherenx x x x

k k n K= + ,ny y y y

k k n K= + , nz

k k n K= +z z z

and 2

x

x

KL

π= ,

2y

y

KL

π= ,

2K

L

π=z

z

.

,x y z

L L and L are lengths of the unit cell along the x, y and z directions. , ,x y

n n nz are the

number of plane waves along the x, y and z directions respectively.

As reported in [28], the attraction of the normalized plane wave approach is the fact that

there is no need to explicitly match the wave function, across the boundary of the barrier and

QD. Hence this method is easy to apply to an arbitrary confining potential problem. As more

plane waves are taken, more accurate results are achieved. We used thirteen normalized plane

waves in each direction to form the Hamiltonian matrix (i.e. , ,x y

n n and nz from −6 to 6) and

we formed 2197*2197 matrix. It was found that using more than 13 normalized plane waves

in each direction takes significantly long computational time and only about 1 meV more

accurate energy eigenvalues. By substituting the Eq. (8) in Schrödinger equation,

eigenfunctions and eigenvalues are calculated. The energy Eigenvalues of the considered

structure have been demonstrated in Fig. 2.


Fig. 2. Energy diagram for the proposed structure and the strongest transition ‘a’.

The transition matrix element, | |if i fr rψ ψ→

=< > can be calculated using obtained wave

functions. In this relation i f

andψ ψ are the initial and final transition states, respectively.

The oscillator strength, if

f , of a given transition is one of the most important factors in the

absorption coefficient ( )α ω and is given by:

( )2

2

2,ifif i f

mf E E r

∗ →

= −ℏ

(9)

where i

E and f

E are the initial and final transition state energy, respectively.

The absorption coefficient can be expressed as [16]:

( )

2

* 22

0

( ) ,d op

i f if

if

N n en n f

m c

πα

εε ω ω

Γ= −

− +Γ

ℏ

ℏ ℏ

(10)

where Γ is the life time broadening which is considered 33 10−× eV . Nd is QD volume

density, op

n is refractive index, 0

ε and ε are the permeability of free space and the medium,

respectively. i

n and f

n are the occupation probabilities of the initial and final states. The

occupation probability can be defined as:

/

,/ /

( ) ( )/

B

B B

E k Tie

niE k T E k T

s te e d f Nd

s tc

ερ ε εε

−=

− − + +∑ ∑ ∫

(11)

where s

E is the quantum dot energy levels, ( )ρ ε is the density of continuum states, B

k is

Boltzmann constant, and t

E is the trap levels. For the low temperatures, 1i

n ≈ and 0f

n ≈

and the specific transition is high, but with increasing the temperature, the carriers

redistributed and the transition decreases.


Fig. 3. The behavior of absorption vs. the photon energy for different GaN QD sizes at T=77 K

(the QD sizes are Lx=Ly, and Lz=3nm).

Figure 3 indicates the behavior of optical absorption of the structure with different QD

size for the transition indicated as “a” in Fig. 2. It is obvious that by increasing the size of

QD, the peak of the absorption increases and there is a red shift which can be related to

increasing of the oscillator strength, and decreasing of the energy levels difference,

respectively.

As long as there are unoccupied excited states available, the electrons in the lower states

can participate in photon induced intraband transitions. However, with further increasing of

the temperature, the electrons occupy the excited states and consequently the absorption

coefficient decreases and dark current increases. It should be mentioned that the strongest

photonic transitions are usually the ones which are energetically directly above each other,

with an s-symmetry to p-symmetry change and in our calculation the “a” transition not only

are in the range of 8-12 µm, but also is a transition from a state with s-symmetry to p-

symmetry.

3. Results and discussion

The insertion of the capping layer is supposed to change the transport properties of the

carriers. In this paper, we introduce a structure, which has a low dark current and high

responsivity and therefore have a good signal to noise ratio. The main parameters of the

detector which discussed in this paper by details are the device responsivity, dark current and

detectivity.

3.1 Responsivity

The responsivity is one of the most important parameters of the photodetectors and defined as

the ratio of its output electrical signal, either a current out

I or a voltageout

V , to the input

optical signal. It is given by

,e

R gηω

=ℏ

(12)

Where g is the gain and defined as the ratio of the recombination time over the transit time:


be

Fg

LC

µ=

where Cbe is the quantum mechanical capture rate into the QD excited state. Estimates for the

Cbe in the literatures, are in the range of 11 12~ 10 10− Hz for shallow excited states, which are

reachable by acoustic phonon emission, and is about 10~ 10 Hz for deep levels [16]. µ is the

mobility of the electron, which has been successfully demonstrated in our previous work by

considering all scattering mechanisms, and the effects of temperature and electric fields [29].

η is the quantum efficiency and is defined as:

( ) /

( ) /0

( ) ,E F k Tec B

ecE F k Tec B

ec

eL

e

νη α ων ν

−

−

= +

(13)

Here 0

ν is the relaxation rate from the photo excited state to all other states and here

considered 10~ 10 , L is the device length, Eec

is the effective field dependent energy difference

between the photoexcited state and the continuum, ec

ν is phonon assisted escape to

continuum prefactor and expected to have only a weak dependence on the temperature and in

this paper is considered 13~ 10 . The temperature dependent normalized responsivity (R/R0) at

different applied electric field are calculated and plotted in Fig. 4:

( ) /

/ .0 ( )/

0

B

B

E F k Tece

ecR R n Fi E F k Tece

ec

υ

υ υ

− = − +

(14)

Fig. 4. The responsivity of GaN QDIP (the QD sizes are Lx=Ly=10nm and Lz=3nm) as a

function of temperature for different applied electric field.


Fig. 5. The normalized responsivity of GaN QDIP (the QD sizes are Lx=Ly=10nm and

Lz=3nm) as a function of the external electric field for different temperatures.

As shown in Fig. 4 with increasing the temperature until 100-170K the responsivity

increases and further increasing of the temperature decreases the responsivity. To explain this

effects, as can be deduced from the relation (12), there are two main sources for temperature

dependence of the responsivity; current gain, and quantum efficiency. So, the increasing of

the temperature increases the current gain as well the responsivity. With further increasing the

temperature i

n starts to decreasing, therefore the absorption coefficient and quantum

efficiency decreases and it make a reduction in the responsivity.

Also, the logarithm of the normalized responsivity versus applied fields for different

temperatures has been illustrated in Fig. 5. We have not considered the temperature

dependency of the life time 0

1/ ( )Tν and assumed it as a constant. As shown in this figure,

with increasing the applied electric field the responsivity increases. The reason for this

behavior is that the increasing of the applied bias increases significantly the current gain as

well as the photocurrent.

3.2 Dark current

In the absence of any incident light and the existence of applied fields there is an unwanted

electrical current which is well-known as dark current. At high temperature range, the dark

current originates from thermionic emissions and for low temperatures, sequential resonant

tunneling and phonon-assisted tunneling are probably the dominant components of the dark

curve. This important parameter has been discussed in several articles [4,5,9].

Considering the most important factors dark current could be written as [30]:

3/ 2 1/ 20 / / /

1/ 2/

.(1 )

11(1 )

E k T E eFa E eFa k Tsc B sc sc Bs s

scE eFa k Tsc ss sc B

e be

Ae FN g f e e e edIdark n C w fe be e e

n C

ς ς

ς

µυ

− − −

−

−= ×

−−+

−

∑ (15)

In this relation s

f is the Fermi function for the s'th energy state, A is the illuminated area of

the device, (1 )e

n− is the probability that the state is empty. SC

w is the escape rates between

the band and QD states,

1* 2

2

2e

maς

= ℏ

, and S

g is the density of excited states. Considering


0

0 (1 )

Ae FNd

I gec sn C

e be

µυ=

−, the logarithm of the normalized dark current

0

darkI

I

versus applied bias

for different temperature has been plotted in Fig. 6. As depicted in figure, at low temperature

the dark current increases rapidly as the bias increases. This can be attributed to the fast

increase of electron tunneling between the QDs. With increasing the bias, the electron density

increases in QD and when a large fraction of the QD states are occupied, further increase in

bias does not significantly alter the electron density. This causes a lowering of the energy

barrier for injected electrons at the contact layers, which results in the nearly exponential

increase of the dark current. Also it should be mentioned that the activation energy decreased

linearly with bias. At high bias, the activation energy is close to ~kT, which resulted in high

dark current even at low temperature.

It should be mentioned that the proposed structure has a very low dark current in

comparison to the structures introduced in Ref [16, 30, 31]. It can be deduced from the

relations, where the high values for Cbe not only will decrease the gain, but also the dark

current. So, structures with high densities of QDs might be useful and have a better

performance in suppressing the dark current effects. Having high densities of QDs has

another benefit. It gives hope to engineering the band structure in order to enhance the

tunneling of the photoexcited carriers from the large dots (capping layer). Therefore we can

have high barriers in order to suppress the thermionic term in dark current without having

presentiment about collecting the photoexcited carriers.

Fig. 6. The dark current vs. external electric field for GaN QDIP (the QD sizes are

Lx=Ly=10nm and Lz=3nm) at different temperatures.

3.3 Detectivity

The detectivity is one of the most important factors of detectors and is considered as figure of

merit in most of the literatures. Specific detectivity, is defined as

.R A fresponsivity A

Dnoise i

Nf

∆×∗= =

∆

(16)

In this relation N

i is the noise current and defines as 4i eI g fN d

= ∆ , f∆ is the band width

frequency and we considered it ~ 1 .


As mentioned before, with increasing the temperature the dark current increases, thus the

responsivity decreases and these behaviors will affect the detectivity. For a given applied bias

and temperature, the responsivity in our proposed structure lower than the other structures

without capping layer. On the other hand, the dark current is much lower in our proposed

structure. This may because an enhancement in impact ionization, which is enabled by the

increased operating voltage that results from the lower dark current [15].Consequently, a net

improvement in the signal to noise ratio is expected. The specific detectivity as function of

temperature for different applied bias are shown in Fig. 7.

The results are representative of high values for specific detectivity in compared to

structures which have been studied previously. Xuejun Lu et al reported in [32], Peak specific

photodetectivity D* of 93.8 10× 1/ 2 /cmH Wz and 81.3 10× 1/ 2 /cmH Wz at the detector

temperature T = 78K and T = 170 K, respectively. Zhengmao Ye give an account that for the

photoresponse peaked at 6.2 mm and 77 K for −0.7 V bias, the responsivity was 14 mA/W

and the detectivity, was 1010 1/ 2 /cmH Wz [33]Bhattacharya et all, reported the some deal

high detectivity, about 6 1/28.6 10 /cmH W× z , in 17 mµ wave length for 300k temperature

[34] and in the other work they reported 9 * 1/2 116 10 ( / ) 10D CmH W× ≤ ≤z for

temperatures100 200K T K≤ ≤ [18,22]. Here we present appropriate results in comparison

and the device which introduced has a good potential to be compared with the structures, have

been presented in [34,35].

Fig. 7. The peak of specific detectivity versus temperature for GaN QDIP

(the QD sizes are Lx=Ly=10, and Lz=3nm) in applied bias of 1 V.

4. Conclusion

As stated in this paper and detailed in numerous publications, owing to their unique material

characteristics, III-N QDIPs have the potential for superior performance as infrared detectors

in the LWIR. In this article we report on the photodetector characteristics related to QDIPs.

The amount of the dark current which calculated is exceptionally perfect and the specific

detectivity of the devise is appreciable in high temperatures, even room temperature.

Therefore the proposed structure will be considered a proper alternative to the mature

technologies that have been widely deployed. The structure studied is sufficiently general, so

covers a large rang of possible device types. Due to better 3-D confinement of carriers,


operating in higher temperatures was observed. High density of QDs was suggested to solve

the collecting difficulties of the carriers. Also the results indicate that there is hope for band

structure engineering for further improve of the detector parameters at high temperatures.


High performances III-Nitride Quantum Dot infrared photodetector operating at room temperature

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