InP(001)

Materials Science and Engineering C 26 (2006) 583 – 585 www.elsevier.com/locate/msec

Short communication

Capacitance–voltage profile characteristics of Schottky barrier structure with InAs quantum dots grown on InAlAs/InP(001) M. Baira a, R. Ajjel a,*, H. Maaref a, B. Salem b,1, G. Bre´mond b, M. Gendry c, O. Marty d a b

Laboratoire de Physique des Semiconducteurs et des Composants Electroniques, De´partement de Physique, Faculte´ des Sciences, 5019 Monastir, Tunisia Laboratoire de Physique de la Matie`re-LPM (UMR-CNRS 5511), INSA de Lyon, Baˆtiment Blaise Pascal, 7 Avenue J. Capelle, 69621 Villeurbanne, France c Laboratoire d’Electronique, Optoe´lectronique et Microsyste`mes-LEOM (UMR-CNRS 5512), Ecole Centrale de Lyon, 36 Avenue G. de Collongue, 69134 Ecully, France d Laboratoire d’Electronique-LENAC, Universite´ Lyon 1, F-69622 Villeurbanne, Lyon, France Available online 28 November 2005

Abstract Capacitance – voltage, C(V) studies have been carried out on Schottky barrier structure containing a sheet of self-organized InAs quantum dots (QDs) grown on InAlAs lattice matched to InP in order to deduce the electrical properties of the QDs. Three electron levels have been detected in n-type material, and were attributed to the s ground, the p excited, and the d excited states. Some parameters of the structure, such as the position of the InAs QD plane, the electron concentration in the QDs and an approximate QD height were deduced from the C(V) profile analysis. These results are in good agreement with the transmission electron microscopy (TEM) study realized on the structure. D 2005 Elsevier B.V. All rights reserved. Keywords: Capacitance – voltage profile; Quantum dots; InAs/InAlAs/InP structure

1. Introduction Quantum well infrared photodetectors (QWIPs) utilizing intersubband transitions have been widely investigated during the last several years due to clear commercial and military applications [1]. As predicted theoretically [2], quantum-dot infrared photodetectors (QDIPs) can surpass QWIPs. Compared to QWIPs, QDIPs can have lower dark current and higher photoelectric gain, besides also being intrinsically sensitive to normally incident radiation [2]. A number of research groups have reported fabrication and experimental studies of QDIPs [3– 5]. In spite of their potential for the realization of QDIPs [6 – 9], very little is known about the electronic properties of InAs QDs grown on an InAlAs barrier layer lattice matched to InP substrate. In this paper, we focus our interest to the study of the electronic and the structural characteristics of these QDs. C(V) studies have been carried out on Schottky barrier structure containing a sheet of self-organized InAs QDs grown on InAlAs * Corresponding author. E-mail address: [email protected] (R. Ajjel). 1 Present address: De´partement de Physique, Universite´ de Sherbrooke, Sherbrooke, Que´bec, Canada J1K 2R1. 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.10.003

lattice matched to InP in order to deduce the electrical properties of the QDs. Three electron levels have been detected in n-type material, and were attributed to the s ground, the p excited, and the d excited states. Some parameters of the structure, such as the position of the InAs QD plane, the electron concentration in the QDs and an approximate QD height were deduced from the C(V) profile analysis. These results are in good agreement with the transmission electron microscopy (TEM) study realized on the structure. 2. Results and discussions The investigated sample was grown on an (001)-oriented InP substrate by solid source molecular beam epitaxy (SSMBE). It consists of a 0.4-Am-thick In0.52Al0.48As (InAlAs) buffer followed by 0.9 nm of InAs. The InAlAs buffer surface preparation and the InAs growth conditions were adjusted to favor dot-like islands [10]. The InAlAs buffer layer was Sidoped at 3
1016 cm 3 (n-type material). The InAs thickness of 3 monolayers (ML) is just above the 2D/3D growth mode transition detected by reflection high electron energy diffraction at 2.5 ML. Then, a 10-nm-thick non-intentionally doped (n.i.d.) layer of InAlAs, followed by a 170-nm-thick Si-doped

M. Baira et al. / Materials Science and Engineering C 26 (2006) 583 – 585

NCV ¼ 

C3 eA
 ; and W ¼ dC C qA2 e dV

ð1Þ

where A denotes the Schottky barrier area, q the elementary charge, and ( is the dielectric constant. It reveals a first peak at a value of W corresponding to the depth of the plane of the QDs, and a second peak shifted of about 0.13 Am. The theoretical fit of the capacitance is obtained using a simple model based on the definition of the capacitance   dQ C ¼ dV [12] where the charge is given by the integral of the density of states and the energy distribution. For the calculations, we assumed that the Fermi level in dots was the same as in the highly doped substrate [13]. We have solved numerically Eq. (3) using the energy level E ei, the energy dispersion DE ei and N dot as fitting parameters. The fitting process was limited to the voltage range from 0 V to  3.5 V, where the contribution of the dots dominates the capacitance of the structure. the theoretical fit reproduces the

1.1x10-9

T =300K

1.0x10-9

1.6x10

NCV (cm-3)

InAlAs layer and a 20-nm-thick n.i.d. InAlAs layer, were grown. Schottky contacts were fabricated by evaporating 20 nm Au onto InAlAs through a contact mask. The back ohmic contact was formed by depositing AuGe/Ni/Au followed by rapid thermal annealing at T = 300 -C. The C(V) characteristics were measured under a frequency of f = 1 MHz with a 15-mV measuring signal. Fig. 1 shows a cross-sectional transmission electron microscopy (TEM) image. The inset reveals the form and the dimensional of one dot. The heights of the dots are in the range of (3.3 – 4.5) nm. This image depicts the existence of a wetting layer. This layer, always present in the Stranski – Krastanow growth mode, is a thin stained quantum well with a thickness of about 1 ML on the top of which dots are formed. The electrical results were based on an analysis of the C(V) measurements. Fig. 2 shows the C – V measurements performed at T = 300 K for the InAs QDs grown on n-type InAlAs/InP. The C(V) characteristics present two plateaux indicating filled electron states in the QD plane [11]. The inset of Fig. 2 shows the concentration N C – V as a function of the space charge layer width W. It was obtained from the expression:

Capacitance (F)

584

9.0x10-10 -10

8.0x10

17

1.2x1017 8.0x1016 4.0x1016 0.15 0.20 0.25 0.30 0.35 0.40 Depletion width (µm)

-10

7.0x10

-10

6.0x10

Fitted bulk capacitance Experimental results Theoretical fit

-10

5.0x10

4.0x10-10 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Reverse bias (V) Fig. 2. C(V) measurements performed at T = 300 K and the theoretical fitting for the InAs QDs embedded in a n-type InAlAs matrix. The inset shows the concentration N C – V , deduced from Eq. (1), as a function of the space charge layer W.

experimental behavior and the fitted parameters of the zerodimensional Gaussian distribution were found to be E e1 = 240 meV with a degeneracy g 1 = 2, E e2 = 180 meV with g 2 = 2, E e3 = 70 meV with g 3 = 6 , and DE e = 40 meV were found. In addition, the total number of QDs was found to be N dot = 4.5
1010 cm 2. E e1 , E e2, and E e3 were attributed, respectively, to the s ground, the p excited, and the d excited electron states as it is deduced by recent studies [18]. Under the assumption that there is no particular interactions between the dots and between the bulk material and the quantum dots, the charge contributing to the capacitance is the sum of free three-dimensional electrons or holes in the InAlAs surrounding the dots and by the confined electrons or holes in the dots. The density of charge is given by the following expression [14]:   Z qðzÞ ¼ q NCV W  Dð E; Vdot Þxf ð E; Vdot ÞdE

ð2Þ

where D(E,V dot) is the density of states, f(E,V dot) is the Fermi – Dirac energy distribution, and V dot is the voltage across the quantum dots.

20 nm

1.8x10

17

1.6x10

17

Theoretical fit Experimental results

NCV (cm-3)

1.4x1017

6 nm α = 15º

1.2x10

17

1.0x1017 8.0x1016 6.0x1016

H = 3.9 nm

4.0x10

16

0.15 25 nm

Fig. 1. Cross-sectional TEM image for the InAs QDs grown on InAlAs/ InP(001). The inset shows the form and the dimensional of one dot.

0.20

0.25

0.30

0.35

0.40

Depletion width (µm) Fig. 3. The experimental and the theoretical fitting of the concentration N C – V as a function of the space charge layer W for the n-type material.

M. Baira et al. / Materials Science and Engineering C 26 (2006) 583 – 585

The integration of the Poisson equation from the surface to the interior of the semiconductor can be written as: V  /B ¼

 qNCV W 2 2es Z qVdot Dð E; Vdot Þxf ð E; Vdot ÞdE þ es

ð3Þ

where / B is the Schottky barrier height. L dot is the nominal position of the dots plane measured from the semiconductor surface. The differentiation of this equation with respect to the bias can be written as:    es qLdot DV ¼  qNCV W 2 DCbulk þ es Cbulk Z  d Dð E; Vdot Þxf ð E; Vdot ÞdE dV
ð4Þ dV Then, N CV can be written as: 
  C3 dCbulk 1 qLdots Dn NCV ¼ bulk 1þ qes dV es DV

on InAlAs lattice matched to InP in order to deduce the electrical properties of the QDs. Three electron levels have been detected in n-type material, and were attributed to the s ground, the p excited, and the d excited states. Some parameters of the structure, such as the position of the InAs QD plane, the electron concentration in the QDs and an approximate QD height were deduced from the C(V) profile analysis. These results are in good agreement with the transmission electron microscopy (TEM) study realized on the structure. References [1] [2] [3] [4] [5] [6]

ð5Þ

[7]

This procedure was used also by Chiquito et al. [15] in the case of InAs/GaAs QDs. The results of calculations for the n-type material are represented in Fig. 3. The full width at half maximum is related to the spatial resolution of the C(V) profile along the growth condition as it is deduced by Schubert et al. [16]. We can use the profile width to evaluate the effective electronic radius which can be considered as an estimate of the height of the selfassembled dots and the density of electrons localized in the dots. We have found an average value of about 4 nm for the C(V) profile width in good agreement with TEM results. On the other hand, Schubert [17] proposed that the density of electrons in the plane containing the dots can be obtained from the C(V) profile by the integration of the N CV xW curves. Using this procedure, we have obtained an average value of the density of electrons about 1011 cm 2. We note that at T = 300 K, an additional quantity of electrons contribute and then the measured capacitance increases. This is attributed probably to a thermal excitation of the electrons from the barrier.

[8]

3. Summary C(V) studies have been carried out on Schottky barrier structure containing a sheet of self-organized InAs QDs grown

585

[9] [10] [11]

[12] [13] [14] [15] [16] [17] [18]

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