The transition mechanisms of quantum-dot/quantum-well mixed-mode infrared photodetectors

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The transition mechanisms of quantum-dot/quantum-well mixed-mode

infrared photodetectors

Shih-Yen Lin

a,b,c,*

, Shu-Ting Chou

a

, Wei-Hsun Lin

d

a

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan

b_{Department of Photonics, National Chiao-Tung University, Hsinchu 30010, Taiwan}

c_{Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan} d

Institute of Electronic Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

a r t i c l e

i n f o

Article history:

Available online 6 June 2009 PACS:

78.67.Hc 78.30.Fs Keywords:

Quantum-dot infrared photodetectors Quantum dots

a b s t r a c t

The transition mechanisms of a 10-period quantum-dot (QD)/quantum-well (QW) mixed-mode infrared photodetector is investigated in this paper. Both mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) responses are observed for the device. The lower normal incident absorption of the LWIR peak suggests that the QW intra-band transition is responsible for the response while the QD intra-band transition for the MWIR response. Due to the coexistence of MWIR and LWIR responses, the MWIR response should be resulted from one-photon transition while the LWIR response from the two-photon transition. To explain the transition mechanisms of the MMIP device, a model is proposed in this paper. The increases of both MWIR and LWIR responses with increasing measurement temperatures observed for the device are attributed to the increase of electrons in the QW ground state/wetting layer state resulted from the increase of one-photon absorption process with increasing temperatures.

1. Introduction

Lots of effort has been devoted to the development of quantum-dot infrared photodetectors (QDIPs) [1–6]. QDIPs with high responsivities and operation temperatures have been reported by inserting AlGaAs barrier layers[1–3]. The inﬂuence of QD doping density on the operation voltage and normal incident absorption have also been reported [4]. Device structures with p-type doped GaAs layers inserted within have been proposed [5–6]. The thermal images taken by a 256 256 grating-less QDIP focal-plane array (FPA) operated at 135 K have also been demon-strated[7]. However, considering the thermal imaging applications of QDIPs, two major disadvantages are observed for the devices: (a) for most QDIPs, the detection wavelength is limited in the mid-wavelength infrared (MWIR, 3–5

l

m) range and (b) the wafer uni-formity of QD samples is worse than the conventional quantum-well infrared photodetectors (QWIPs). Therefore, to achieve mul-ti-color detection at both MWIR and LWIR ranges, a 10-period QD/QW mixed-mode infrared photodetector (MMIP) is proposed in this paper. Responses at 4.8/12.7 and 5.3/10.3

l

m at positive and negative biases are observed for the device. Compared with the peaks at MWIR range, the lower normal incident absorption

of the LWIR peaks suggests that the QW intra-band transition is responsible for the responses. The QD intra-band transition should be responsible for the MWIR peaks. To explain the transition mech-anisms of the device, a model is proposed. Assuming wetting layer state (EWL), QW ground state (E0,QW) and QW excited state (E1,QW) are available for the electron transition from QD excited state (E1,QD), one-photon transitions (E1,QD–EQW,0) and (E1,QD–EWL) are responsible for the 4.8 and 5.3

l

m responses at positive and nega-tive biases. The two-photon transitions (E0,QW–E1,QW) and (EWL– E1,QW) are responsible for the 12.7 and 10.3

l

m responses. In this case, the energy difference between the two peaks at either MWIR or LWIR ranges would correspond to the same value of (E0,QW– EWL). The similar energy differences 24.4 and 22.8 meV at MWIR and LWIR ranges have conﬁrmed the transition model.

The sample discussed in this paper is grown on (1 0 0)-oriented semi-insulating GaAs substrates by Riber Compact 21 solid-source molecular beam epitaxy system. The sample structure is shown in Table 1. With 300 and 600 nm n-type GaAs layers doped to 2 1018_cm 3_{as the top and bottom contact layers, the 10-period} InAs/GaAs/Al0.2Ga0.8As structures were grown as the active region. For each period, 1 nm undoped GaAs/2.4 ML InAs QDs/8 nm n-type GaAs QD/QW structures are sandwiched between two 30 nm Al0.2Ga0.8. As barrier layers. The doping density at the QW region is 1 1018cm 3 for the device. After mesa formation and metal evaporation, 100 100

l

m2 _{devices were fabricated for} measurements. The spectral responses were measured under an

*Corresponding author. Address: 128 Sec. 2, Academia Road, Nankang, Taipei 11529, Taiwan. Tel.: +886 3 5744364; fax: +886 3 5745233.

E-mail address:[email protected](S.-Y. Lin).

Infrared Physics & Technology 52 (2009) 268–271

Contents lists available atScienceDirect

Infrared Physics & Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i n f r a r e d

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edge-coupling scheme. For this purpose, the devices were polished 45°_{-off at one side of the samples. The infrared light was normally} incident to the polished surface. The applied voltages to be positive or negative were deﬁned according to the voltage polarity applied to the top contact. The measurement system for spectral response consists of a Spectral 100 Fourier transformation infrared (FTIR) spectroscopy coupling with a Janis cryostat and a current pream-pliﬁer. The current–voltage (I–V) characteristics were measured by using the Keithley 236 source measure unit[4–6].

The 10 K spectral responses of the device at ±2.8 V are shown in Fig. 1a. As shown in the ﬁgure, 4.8 and 12.7

l

m responses are ob-served for the device under 2.8 V, while 5.3 and 10.3

l

m responses are observed under 2.8 V. The results suggest that there are four different transition mechanisms involved in the spectral response measurements. The 10 K spectral responses of the device at ±2.0 V are shown inFig. 1b. As shown in the ﬁgure, both 4.8 and 5.3

l

m responses at MWIR range are observed for the device at lower applied voltages, while no responses are observed at the LWIR range. The results suggest that both transition mechanisms at the MWIR range would occur at the same time for the device.

Dominant responses 4.8 and 5.3

l

m are observed at positive and negative biases, respectively. The coexistence of the two peaks at 12.7 and 10.3

l

m is not observed from the spectral responses at either ±2.8 or ±2.0 V. The results suggest that the transition mech-anisms of the peaks at the LWIR range is different from those at the MWIR range.

To further investigate the transition mechanisms of the device, responses of the device measured under IR light irradiation with different polarizations are performed[4–5]. The measurement con-ﬁguration is shown in Fig. 2a. The normalized MWIR and LWIR responsivities of the device measured under the IR light irradiation with different polarizations at 2.8 and 2.8 V are shown inFig. 2b and c, respectively. Compared with the values obtained under p-mode IR light irradiation, the MWIR/LWIR responsivities under s-mode IR light irradiation are reduced to 66/40% and 65/40% at 2.8 and 2.8 V, respectively. The results suggest that although dif-ferent transition mechanisms are responsible for the four peaks

ob-2 4 6 8 10 12 14 0.00 0.02 0.04 0.06 0.08 12.7 μm 10.3 μm 4.8 μm 10 K 2.8 V -2.8 V

Wavelength (

μ

m)

Responsivity (A/W)

5.3 μm 2 4 6 8 10 12 14 0.000 0.005 0.010 0.015 0.020 0.025

Responsivity (A/W)

Wavelength (

μ

m)

2.0 V -2.0 V 10 K

(b)

(a)

Fig. 1. The 10 K spectral responses of the device at (a) ±2.8 V and (b) ±2.0 V.

45°

Substrate

Top Contact

Bottom Contact

Ceramic Pad

s

p

θθ

Device

0 20 40 60 80 100 0.0 0.2 0.4 0.6 0.8 1.0 0.4 0.66 Cos2θ MWIR response LWIR response

θ

(degree)

Normalized Responsivity

10 K 2.8 V 0 20 40 60 80 100 0.0 0.2 0.4 0.6 0.8 1.0

Normalized Responsivity

0.4 0.65 Cos2θ MWIR response LWIR response

θ

(degree)

10 K -2.8 V

(c)

(b)

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Fig. 2. (a) The measurement conﬁguration of polarization-dependent responses of the device and the normalized responsivities of the device under IR light irradiation with different polarizations at (b) 2.8 and (c) 2.8 V.

Table 1

The wafer structure of the 10-period MMIP.

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served at positive and negative biases, the MWIR responses at 4.8 and 5.3

l

m should be resulted from QD intra-band transitions while the LWIR responses 12.7 and 10.3

l

m from QW intra-band transitions[4]. It seems to be contradictory that both MWIR and LWIR responses would be observed for the device at the same ap-plied voltage. The reason is that no empty states at the QD excited state would be available for electron transitions if the QW ground state is ﬁlled with electrons. In this case, it is reasonable to assume that one-electron/one-photon process is for the MWIR responses while one-electron/ two-photon process is for the LWIR responses. The observation of the LWIR responses is resulted from the re-excitation to the QW excited state of photo-excited electrons transited from the QD ground state.

To further explain the results and arguments described above, a model is proposed. A simplified schematic band diagram of the de-vice under positive and negative biases are shown inFig. 3. It is as-sumed that five states are in the structure, which are ground states and first excited states in both the QD and QW regions denoted as E0,QD, E1,QD, E0,QW, and E1,QW, and the wetting layer state denoted as EWL. According to the observations of polarization-dependent spec-tral responses, the responses at 4.8 and 5.3

l

m should be resulted from the QD intra-band transitions while the 12.7 and 10.3

l

m re-sponses from QW intra-band transitions. Assuming the E1,QD is fully occupied with electrons due to the n-type doping in the QW region, transitions between E0,QD and other higher-order states would be less possible due to the large energy difference in-be-tween. Therefore, two transition mechanisms (a) and (b), as shown inFig. 3, should be responsible for the 4.8 and 5.3

l

m responses of the device, where (a) represents the E1,QD–E0,QWtransition and (b) the E1,QD–EWLtransition. Due to electron wave function of EWLis similar to that of E0,QW, it is reasonable to assume that the normal incident absorption ratio should be similar for both transitions of (a) E1,QD–E0,QWand (b) E1,QD–EWL[8]. When an external voltage is applied to the devices, both transitions (a) and (b) would occur. However, when the device is under positive biases, as compared to the photon-excited electrons at E0,QW, the phonon-assisted

tran-sition to the E0,QWstate would be necessary for the electrons at EWL prior tunneling through the AlGaAs barrier layer. In this case, tran-sition (a) would be dominant for the devices positively biased such that the 4.8

l

m response would be observed. For the devices neg-atively biased, considering the small energy difference between E0,QWand EWL, the tunneling probability for electrons at the two states should be similar. However, considering the smaller energy between E1,QDand EWL, the absorption coefﬁcient of transition (b) should be higher than that of transition (a). In this case, transition (b) would be dominant for the devices negatively biased such that the 5.3

l

m response would be observed. Assuming that the Fermi level of the device are lower than the EWLstate, the observed LWIR response of the device is attributed to the two-photon absorption with EWLand E0,QWas the intermediate states[9]. Therefore, dom-inant transitions of (c) E0,QW–E1,QWand (d) EWL–E1,QWwould be ob-served for the device at positive and negative biases, respectively. In this case, when the device is positively biased, 12.7

l

m response would be observed while 10.3

l

m response is observed at negative biases. Another evidence supporting this argument is that the en-ergy difference between 12.7 and 10.3

l

m is 22.8 meV, which is very close to the energy difference 24.4 meV between 4.8 and 5.3

l

m. The result is consistent with the prediction of the model that the same energy state difference (EWL–E1,QD) is responsible for the peak energy differences at either MWIR or LWIR ranges.

To investigate the temperature dependence of responsivities for the device, the 10 and 77 K spectral responses of the device mea-sured at 2.6 V are shown in Fig. 4. As shown in the ﬁgure, in-creases of responsivities at both MWIR and LWIR regions with increasing temperatures are observed for the device. The phenom-enon is quite different from the invariant photocurrents of conven-tional QWIP or superlattice infrared photodetectors (SLIP)[10]. For the conventional QWIP devices, considering the small temperature measurement range (10–77 K), no signiﬁcant change on electron capture probability in the QW region would be observed. In this case, invariant photocurrents with increasing temperatures would be observed for the device. However, for standard QDIPs, the elec-tron capture probability in the QD structure would rapidly de-crease with increasing temperatures [11]. In this case, an increase of photocurrents with increasing temperature would be observed for the device. In the case of MMIPs, the MWIR response of the device is resulted from the QD intra-band transitions. The MWIR response of the device would increase with increasing tem-perature as in the case for standard QDIPs. As for the LWIR re-sponses, an increase in the MWIR response would represent increasing electron occupancy in the intermediate states like EWL and E0, QW. In this case, the transition probability in the QW struc-ture would also increase as in the case in the QD strucstruc-ture. The characteristic would be quite helpful for the development of

E_0,QD E_1,QD E_WL E_0,QW E_1,QW E_0,QW E_1,QW E_0,QD E_1,QD E_WL Positive Bias Negative Bias Growth Direction (c) (a) (b) Electron Photocurrents Electron Photocurrents (d)

Fig. 3. A simpliﬁed schematic band diagram of the device under positive and negative biases. 2 4 6 8 10 12 0.00 0.02 0.04 0.06 0.08 0.10

10 K

77 K

10 K

-2.6 V

Wavelength (

μ

m)

Responsivity (A/W)

Fig. 4. The 10 and 77 K spectral responses of the device at 2.6 V. 270 S.-Y. Lin et al. / Infrared Physics & Technology 52 (2009) 268–271

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high-temperature operation infrared photodetectors in the LWIR range.

In conclusion, a 10-period QD/QW MMIP is proposed in this pa-per. Responses at 4.8/12.7 and 5.3/10.3

l

m at positive and negative biases are observed for the device. Compared with the peaks at MWIR range, the lower normal incident absorption of the LWIR peaks suggests that the QW intra-band transition is responsible for the responses. The QD intra-band transition should be respon-sible for the MWIR peaks. A model is proposed to explain the tran-sition mechanisms of the device. While the one-photon trantran-sition process is responsible for the MWIR responses, the LWIR responses should be resulted from a two-photon process. The increases of both MWIR and LWIR responses with increasing measurement temperatures are also observed for the device. The results are dif-ferent with the invariant photocurrents of QWIPs with increasing temperatures. The phenomenon is attributed to the increase of electrons in the QW ground state/wetting layer state resulted from the increase of one-photon absorption process with increasing temperatures. According to the results of this paper, multi-color detection in both the MWIR and LWIR ranges could be achieved within a single MMIP structure by properly designing the structures.

Acknowledgements

This work was supported in part by the National Science Coun-cil, Taiwan under Grant Numbers NSC 96-2221-E-001-030 and NSC 96-2218-E-002-012.

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