Electric field dependence of intersubband transitions in GaAsAlGaAs single quantum wells

applied

surface science ELSETIER

Applied Surface Science 113/l

14 (1997) 90-96

Electric field dependence of intersubband transitions in GaAs/AlGaAs single quantum wells A. Mathur, Y. Ohno, F. Matsukura, K. Ohtani, N. Akiba, T. Kuroiwa, H. Nakajima, H. Ohno * Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku University, 2-1-I Katahira. Aoba-ku, Sendai 980-77, Japan

Abstract The effects of applied electric fields on intersubband (ISB) transitions in modulation-doped n-i-n and p-i-n type GaAs/AlGaAs single quantum well (SQW) structures are studied experimentally using Fourier Transform InfraRed (FTIR) spectroscopy. The n-i-n SQW devices exhibit a red shift of 0.7 meV in the ISB transition energy and a 12% decrease in the integrated absorbance at the maximum applied bias of 3.2 V with reference to their zero bias values, most of which is attributed to a rise in the device temperature due to Joule heating owing to the current flowing through the device. The p-i-n SQW devices exhibit strong quenching of the absorption when a negative bias is applied, while there was no observable shift in the ISB transition energy. PACS: 73.4O.K~; 78.66.-w Ke.ywords: Intersubband

transitions;

Quantum

wells; FTIR spectroscopy;

1. Introduction Ever since the first direct observation of an intersubband (ISB) transition within the conduction band of a GaAs quantum well (QW) by West and Eglash [l], tremendous progress has been made, not only in understanding the physics of ISB transitions, but also in the realization of novel devices such as far-infrared (FIR) photodetectors, modulators and the unipolar laser [2]. FIR optical devices based on ISB transitions offer unique technological advantages

* Corresponding author. [email protected].

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Molecular

beam epitaxy

which include the use of mature materials and processing techniques, for example the GaAs/AlGaAs system, rather than low bandgap materials like HgCdTe; achieving large wavelength tunability within the same material system and optoelectronic integration [3]. An important consideration from both physical and device technology viewpoints is the effect of applied electric fields normal to the QWs on ISB transitions. While a number of experimental studies have addressed this issue [4,5] in multiple quantum well (MQW) structures, relatively few studies have been directed towards single quantum well (SQW) structures. SQWs are attractive candidates for device applications due to their superior optical character-

0 1997 Elsevier Science B.V. All rights reserved.

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A. Mathur et al. /Applied Surface Science I13 / I14 (1997) 90-96

istics. For example, doped SQW far-IR photodetectors have an exceptionally high optical gain as compared to MQW detectors [6]. Furthermore, it is expected that the electric fields would be more uniform across a SQW as compared to those across relatively thick MQW structures, where high- and low-field domains are known to be formed [3]. In this paper, we present an experimental study on the effect of applied electric fields on ISB transitions of electrons in modulation-doped GaAs/AlGaAs SQW with n-i-n and p-i-n structures. The n-i-n SQW structure enables us to apply bias voltages uniformly across the quantum well, while the p-i-n SQW structure allows the control of carriers (via injection or depletion) in the quantum well upon the application of bias without metallization on the entire sample surface as required in Schottky-diode based carrier control schemes [7].

2. Experimental procedures The SQW structures were grown on (001) semiinsulating GaAs substrates by molecular beam epitaxy (MBE). The n-i-n SQW samples consist of an 800 nm GaAs:Si (1 X lOi’ cm-3) for the bottom contact, a 300 nm GaAs buffer layer, a 9 nm GaAs QW clad on either side by 35 nm thick AlGaAs barriers (the center 15 nm of which were Si-doped to 1 X lo’* cm- ‘), a 300 nm of undoped GaAs, and a 300 nm cap layer of GaAs:Si (1 X 10” cmm3) for the top n’ contact. The p-i-n SQW sample consists of an 800 nm GaAs:Si (5 X 10” cme3) for the bottom n+ contact, a 50 nm undoped AlGaAs, a 10 nm AlGaAs:Si (1 X 10” cmm3), a 5 nm AlGaAs spacer layer, an 8 nm GaAs QW, a 5 nm AlGaAs spacer, a 15 nm AlGaAs:Si (1 X 10” cmm3>, a 50 nm AlGaAs, a 100 nm GaAs, and a 300 nm cap layer of GaAs:Be (1 X lOI crnm3) for the top p+ contact. Typical growth temperature was about 560°C and the aluminum mole-fraction in AlGaAs alloy was set to be 0.3 + 0.02. A waveguide geometry for the SQW samples was used to enhance the ISB absorption by multiple internal reflection (MIR) of the IR radiation within the sample. 45” facets were polished on the far edges of the (10 mm X 7 mm) samples used for the devices to enable coupling of the IR radiation into the sam-

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ple. In order to apply uniform electric fields across the SQW structures, an interdigitated electrode pattern was utilized for the top and bottom contacts, as shown in Fig. 1. For both the n-i-n and p-i-n type of samples, a comb-like mesa was first formed using standard wet-etching techniques. Subsequently, for the n-i-n samples Au/Ni/AuGe/Ni electrodes were evaporated in a single-step on both top and bottom of the mesa for n-type Ohmic contacts. For the p-i-n sample, Au/Ni/AuGe/Ni electrodes were first evaporated on the bottom of the mesa for the n + Ohmic contact, and in the following step, Cr/Au electrodes were evaporated on the top of the mesa for the p+ contact. The IR absorption spectra was measured using a BIORAD FIS- 175 FTIR spectrometer incorporating an optical assembly to facilitate coupling IR radiation into the waveguide sample structure. Measurements were made at room temperature with a typical resolution of 2 cm-’ (0.248 meV) using a liquid nitrogen cooled MCT detector. In order to achieve high signal-to-noise ratio for the FTIR spectroscopy measurements, the anisotropic character of ISB absorption [ 1I was used to obtain the absorbance spectrum by defining the absorbance (A) as: A = log,,(l/T), where the transmittance (T) is given by the ratio: T = p - polarized spectrum s - polarized spectrum The integrated

absorbance

x 100 (IA) was simply estimated

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et a/./Applied

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by multiplying the peak absorbance by the full width at half maximum (FWHM) of the absorbance spectra

181. 3. Results and discussion Fig. 2 shows the absorbance spectra for the n-i-n SQW device at various bias voltage. As shown in Fig. 3, the ISB transition energy exhibits a red shift of about 0.7 meV and the integrated absorbance decreases by about 12% at the maximum applied bias of 3.2 V, with respect to the values at zero applied bias voltage. The direction of our observed shift in the transition energy is opposite to that of the Stark shift [4], while it agrees with the red shift observed by Liu et al. [51. However, we observed a significant rise in the GaAs three-phonon absorption peak at about 95 meV as the applied bias voltage increases (see inset of Fig. 2). This indicates that, unlike the devices measured by Liu, the device used here was heated up. Previous experimental studies on the effect of

0.04

114 (19971 90-96

temperature on the ISB transition energy by Covington et al. [9] have shown that the ISB transition energy decreases linearly with the increase in temperature with a rate of approximately 0.02 meV/K. In addition, Loehr and Manasreh [lo] have reported that the temperature-induced red-shift of the ISB transition energy is independent of doping in the barrier or the well. Here we performed a temperature calibration of the GaAs three-phonon absorption feature to estimate the temperature rise in our device at different values of the applied bias voltage. We determined that Joule heating, resulting from current flow through the device, raised the device temperature by about 43 K over the ambient temperature at the maximum of the applied bias voltage of 3.2 V. Extrapolating the results of Covington et al. [9], the estimated red-shift in the ISB transition energy related to temperature effects is also plotted in Fig. 3. It is apparent that the observed red-shift can be mostly attributed to a temperature-induced shift and that the net shift (i.e., the contribution due to the applied electric field) is negligibly small. Fig. 4 shows the results of simulation of a modu-

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Energy [mev] Fig. 2. Curve-fitted absorbance spectra for the n-i-n SQW device showing the red-shift of the ISB transition energy with applied bias voltage. The inset shows the GaAs three-phonon absorption peak at 95 meV indicating device heating at high bias voltage.

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et al/Applied

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Fig. 3. Variation of peak ISB transition energy and the normalized integrated absorbance with applied bias voltage for the n-i-n SQW device. The temperature-induced shift is also plotted on the same scale for comparison, showing that most of the observed red-shift is due to device heating.

an applied electric field. The following points can be inferred from this figure: (a) the magnitude of the Stark shift is very small, even at the field strengths

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A. Mathur et al./Applied

I14 (1997) 90-96

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Energy [mev] Fig. 5. Curve-fitted absorbance spectra for the p-i-n SQW device under different bias conditions. bias values. Note the absence of the GaAs three-phonon absorption feature at 95 meV.

of 30 kV/cm, and (b) the magnitude of Stark shift decreases as the sheet carrier densities increase due the effective screening of the applied field by the I 20 -

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The inset shows the raw spectra at three

electrons; and that for sheet carrier-densities of 1 X lOI* cm-* (which is approximately that of our n-i-n device) the Stark shift magnitude is less than

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SQW device. Linear extrapolation of the at about - 3 V, in excellent agreement with

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0.02 meV for an applied electric field of about 30 kV/cm. These theoretical results agree well with those of Bloss [ 111. Thus, both our theoretical predictions and experimental results show good agreement, in that the effect of applied electric fields on the ISB transition energy is very small. The results of FTIR spectroscopy measurements on the p-i-n SQW device are shown in Figs. 5 and 6. Fig. 5 shows the relative absorbance at different bias values, normalized to the absorbance at -5.0 V applied bias voltage. On application of a positive bias an increase in the absorption is observed as carriers are injected into the quantum well region. Positive bias applied beyond + 1.0 V shows a tendency towards saturation. On applying negative bias, carriers begin to be depleted from the QW region and a strong reduction in the absorption is observed, which approaches saturation at about -5 V. Although the p-i-n SQW structure was designed for complete depletion of carriers at approximately -3 V, Fig. 6 shows that a linear extrapolation of the integrated absorbance in the bias region of (+ 1 V, - 2 V) yields a 100% quenching at an applied bias value of approximately -3 V, which is in excellent agreement with our design value. Thus, though there appears to be a deviation in the rate of depletion of carriers at higher bias voltages (< -2.5 V), it is evident that the level of carriers depleted at - 5 V is the same as that designed for complete depletion at - 3 V. It therefore seems plausible that the origin of the absorption feature at - 5 V, shown in the inset of Fig. 5, is most likely, not due to the incomplete depletion of carriers. It can also be observed from Fig. 5 that there is no appreciable shift in the ISB transition energy over the range of applied bias, and also that the lineshape of the absorbance spectrum is unchanged. Theoretical and experimental studies [3,12] on multiple quantum well structures, however, have shown that the intersubband transition energy exhibits a red-shift when the 2D electron density in the QW decreases. Further experimental work and accurate modeling of the physical processes is thus required to achieve better consistency between theory and experiment as regards SQW structures. Negligible device heating occurred over the entire range of applied bias as was confirmed by monitoring the GaAs three-phonon absorption peak at 95

(1997) 90-96

95

meV. However, at bias voltages beyond -5 V the leakage current in the device dominates and device heating becomes appreciable. The ability of the p-i-n SQW devices to effectively control carriers in the quantum well within a modest range of applied bias ( + 1.O V to - 5.0 V, in this case) is promising for such devices to be used as far-IR optical modulators. It is expected that, with further optimization of the p-i-n structure design, growth and process techniques, the leakage current can be minimized and further improvement in device performance achieved.

4. Conclusions We have experimentally studied the influence of external applied electric fields on the intersubband transitions of electrons in GaAs/AlGaAs single quantum wells with n-i-n and p-i-n type of structures by using FTIR spectroscopy. The n-i-n SQW devices show a small red-shift due to device heating. The p-i-n SQW devices showed almost 100% quenching of the ISB absorption at an applied negative bias of - 5 V. The absence of a substantial shift in the ISB transition energy as the carriers are depleted from the quantum well is in contrast to the observation in multiple quantum well structures.

Acknowledgements The authors would like to thank Professor M. Niwano for valuable discussions on the FTIR measurements. They also gratefully acknowledge the Mazda Foundation, and Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, Japan for partial support of this work.

References [l] L.C West and S.J. Eglash, Appl. Phys. Lett. 46 (1985) 1156. [2] H.C. Liu, B.F. Levine and J.Y. Andersson (eds.), Quantum Well Intersubband Transition Physics and Devices, Vol. 270, NATO AS1 Series (Kluwer Academic, Netherlands, 1994). [3] B.F. Levine, J. Appl. Phys. 74 (1993) Rl. [4] Alex Harwit and J.S. Harris, Jr., Appl. Phys. Lett. 50 (1987) 685.

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[5] H.C. Liu, M. Buchanan, Z.R. Wasilewski and H. Chu, Appl. Phys. Lett. 58 (1991) 1059. [6] K.M.S.V. Bandara, B.F. Levine, R.E. Leibcnguth and M.T. Asom, J. Appl. Phys. 74 (1993) 1826. [7] V. Berger, N. Vodjdani, D. Delacoutt and J.P. Schnell, Appl. Phys. Lett. 68 (1996) 1904. [8] H. Asai and Y. Kawamura, Phy. Rev. B 43 (1991) 4748. [9] B.C. Covington, C.C. Lee, B.H. Lu, H.F. Taylor and D.C. Streit, Appl. Phys. Lett. 54 (1989) 2145.

[IO] J.P. Loehr and M.O. Manasreh, in: Semiconductor Quantum Wells and Superlattices for Long-Wavelength Infrared Detectots, ed. M.O. Manasreh (Artech House, MA, 1993) p. 41. [l l] W.L. Bless, J. Appl. Phys. 66 (1989) 3639. [12] J.P. Loehr and M.O. Manasreh, in: Semiconductor Quantum Wells and Superlattices for Long-Wavelength Infrared Detectots, ed. M.O. Manasreh (Artech House, MA, 1993) p. 45.