AlAs superlattice p-i-n diodes

Microelectronic Engineering 43–44 (1998) 139–145

Barrier thickness dependence of photocurrent spectral intensity in GaAs /AlAs superlattice p-i-n diodes K. Kawasaki, K. Kawashima*, M. Takeuchi, K. Fujiwara Kyushu Institute of Technology, Tobata, Kitakyushu 804, Japan Abstract We have experimentally studied photocurrent (PC) spectral intensity in GaAs /AlAs superlattices (SL) with different barrier thickness [LB 5 2 | 18 monolayers (ML)]. When the LB value is less than 6 ML, the PC spectra reveal clear evidence for the Wannier–Stark localization and the PC intensity shows an initially steep increase with the field (at less than 30 kV/ cm) over a wide spectral range and then saturates at nearly the same level regardless of LB . For thick LB samples, however, the PC intensity is strongly dependent on the LB value, very gradually increases and reaches the saturation level under extremely high field conditions. These results are rigorously explained by considering the photogenerated carriers tunneling and the competition between the recombination lifetime and the tunneling escape time which is strongly varied by changing the barrier thickness.  1998 Elsevier Science B.V. All rights reserved. Keywords: Quantum wells; Semiconductors; Tunneling; Photoconductivity

1. Introduction The vertical tunneling transport of photogenerated carriers in the superlattices (SL) is crucial for ultrafast optical switching of modulator and bistable devices. To study the electroabsorption properties and the vertical transport processes of photogenerated carriers, photocurrent (PC) spectroscopy is widely used. In addition to the two basic processes, the competitive carrier recombination is also important for PC spectral features, because the number of photocarriers which tunnel through the intrinsic SL region would decrease via recombination channels. Recently, interesting physical phenomena which influence the PC spectral features have been observed such as transit time effects [1,2]. The barrier thickness and the width of intrinsic layers are considered as crucial factors to affect the competitive processes of recombination and transport. The barrier thickness dependence on the absorption spectral features has also been investigated to clarify field induced modulation of the absorption oscillator strength [3–5]. However, it is not clear yet how the transit time effects, due to the competition between the tunneling transport and the recombination processes, affect absolute PC intensities. *Corresponding author. 0167-9317 / 98 / $19.00 Copyright  1998 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 98 )00156-7

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In this paper, we investigate barrier thickness dependence on PC spectral intensity in GaAs /AlAs SL p-i-n diodes. In addition to studies of room temperature PC spectroscopic features under the applied electric field, the dependence of PC intensity on the barrier thickness is quantitatively evaluated and explained by considering the photogenerated carriers tunneling. For our study five SL samples were prepared consisting of the same p-i-n diode structure except for a barrier thickness in an intrinsic SL layer. Fig. 1 shows a schematic diagram of the SL sample structure. They were grown on an n-type GaAs (100) substrate by molecular beam epitaxy (MBE). The growth sequence was as follows: (a) 0.2 mm n-GaAs buffer, (b) 1 mm n-Al 0.4 Ga 0.6 As cladding, (c) a nominally undoped 100-period GaAs /AlAs SL (which was further confined by nominally undoped 50 nm Al 0.4 Ga 0.6 As cladding layers), (d) 0.2 mm p-Al 0.4 Ga 0.6 As cladding, and (e) 10 nm p-capping layers. The barrier thickness of the SL samples [1, [2, [3, [4 and [5 is 0.57 nm (2 ML), 1.13 nm (4 ML), 1.70 nm (6 ML), 3.42 nm (12 ML) and 5.17 nm (18 ML), respectively. The nominal well thickness of the SL layers was fixed at 6.26 nm (22 ML). Each sample was processed into a 400 3 400 mm 2 mesa using wet chemical etching by standard photolithography techniques to form a p-i-n diode, and then a window-shaped Au electrode was deposited on the p-GaAs capping layer to allow electric field application and optical access. Most of the PC spectra were measured at room temperature using a halogen lamp and a monochromator for excitation and a lock-in-amplifier for a.c. detection. For some SL samples PC spectra were also measured at 17 | 300 K in a temperature controled He cryostat. Fig. 2 shows PC spectra of sample [1 (whose barrier thickness is the narrowest: LB 5 0.57 nm) as a function of applied bias voltage (Vb ) at room temperature. Here the negative value of Vb means the reverse bias to the diode. It is clear from changes of the absorption edge near 1.5 eV by the field that the miniband is formed under the flat-band or low electric field conditions (Vb 5 1 1.0 | 0.0 V) and that the Wannier–Stark localization occurs by the intense field application. Using the Kronig–Penney model within an effective mass approximation the electron miniband width is calculated to be 52.0 meV. The experimental shift of the leading edge amounts to 29.3 meV, which is compared with a half value of the transition miniband width between the lowest electron and heavy-hole states. In the intermediate field range the electron wavefunction is localized into several wells and the Stark ladders [6] are formed, so that the 0th and 61st order Stark ladder transitions are clearly observed at the Vb

Fig. 1. Schematic diagram of the SL p-i-n diode structure.

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Fig. 2. Photocurrent spectra of GaAs /AlAs (6.26 / 0.57 nm) SL diode (sample [1) as a function of applied bias voltage at room temperature. Upward and downward arrows indicate transition peaks of the 0th order and 61st order Stark ladders, respectively, associated with the localized heavy-hole state. The spectra are vertically shifted for clarity.

range of 2 0.5 | 2 6.0 V. As indicated by upward arrows in Fig. 2 the 0th order Stark ladder transition peak appears at 1.521 eV ( l 5 815.0 nm) when Vb exceeds 2 2 V. The 61st order transitions, as indicated by downward arrows in Fig. 2, are seen at the low or high energy side of the leading peak. They shift to the lower or higher energy side by increasing the electric field as expected for the indirect Stark ladder transitions. Electric field induced energy shifts of the Stark ladder transitions are plotted in Fig. 3. The slopes of the energy shifts obtained from the experiment data are in excellent agreement with those calculated using the transfer-matrix (TM) method [7] assuming the build-in voltage (Vbi ) of 1.5 V. When the electric field is further increased (Vb , 2 8 V), the 0th order transition shows an additional shift to the low energy side by the quantum confined Stark effect (QCSE) [8]. Therefore, the Wannier–Stark localization and the QCSE are coexisting in sample [1

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Fig. 3. Stark ladder transition energies (triangles) plotted as a function of electric field for GaAs /AlAs (6.26 / 0.57 nm) SL diode (sample [1) observed at room temperature. Solid lines indicate the calculated transition energies.

because the well width is relatively wider. In addition to the transition peak associated with the ground subband states (electron / heavy-hole: 1e–1h and electron / light-hole: 1e–1l), we note that two excitonic peaks are also observed at 1.578 eV ( l 5 785.7 nm) and 1.672 eV ( l 5 741.5 nm) as the field increases to Vb 5 2 20 V. These peaks are assigned to excitonic transitions between the 1st electron and 2nd and 3rd heavy-hole states (1e–2h, 1e–3h). The 1e–2h and 1e–3h excitonic transitions are theoretically expected to appear at 1.586 eV ( l 5 781.7 nm) and at 1.689 eV ( l 5 734.1 nm), respectively, in agreement with the observed results. Fig. 4 shows PC spectra of sample [3 (LB 5 1.70 nm) as a function of bias voltage at room temperature. Since the LB value is larger than that of sample [1 and the electron miniband width is narrower (5.4 meV), the PC spectral characteristics are similar to those of multiple quantum wells. At a first glance it is obvious that the PC spectra of sample [3 (shown in Fig. 4) are sharper at the 1e–1h and 1e–1l exciton resonances in comparison with those of sample [1 (see Fig. 2). This is because the quantum confinement effect and the resultant excitonic effect are stronger when the LB increases. The 1e–1h and 1e–1l excitonic transitions are located at 1.521 eV ( l 5 815.0 nm) and 1.550 eV ( l 5 799.9 nm) under the low applied field conditions. These values are in excellent agreement with the 0th order ladder transitions in sample [1. They shift to the low energy side with further increase of the electric field by QCSE. For sample [3, two peaks were also observed in Fig. 4 at 1.580 eV ( l 5 784.7 nm) and 1.680 eV ( l 5 738.0 nm) when Vb 5 2 20 V. These peaks are assigned to the 1e–2h and 1e–3h excitonic transitions as in the case of sample [1. It is noted that the 1e–2h and 1e–3h peaks in sample [1 are stronger than those of sample [3. This is explained by considering that the electron tunneling through the thin barrier is more efficient in sample [1 so that the electron wavefunction asymmetry is more enhanced under the applied field that results in permitting the symmetry forbidden transitions. For sample [2, the electron miniband width is calculated to be 17.1 meV and the similar Stark ladder transitions are observed in PC spectra whose characteristics are between those of samples [1 and [3.

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Fig. 4. Photocurrent spectra of the GaAs /AlAs (6.26 / 1.70 nm) SL diode (sample [3) as a function of applied bias voltage at room temperature. The spectra are vertically shifted for clarity.

PC spectra of thick barrier samples [4 and [5 were measured as a function of bias voltage at temperatures of 300–17 K. In comparison with the thin barrier samples whose PC intensity shows an initially steep increase with the field (Vb 5 1 1.0 | 0.0 V) the PC intensity of samples [4 and [5 show completely different behavior. It is found for thick barrier SL samples that PC intensities are very small under the low field regime and gradually increase with the field. Furthermore, PC spectra of sample [5 show a distinctive shape under the low electric field, which does not always reflect electroabsorption features. Instead, negative dips are observed at the lowest 1e–1h and 1e–1l exciton resonances in the weak field PC spectra. When the field is increased, these dips are flattened and then converted into the usual positive PC peaks when the Vb is decreased below 2 12 V. When the sample temperature is decreased, it is found that these negative dips in the PC spectra are significantly enhanced, indicating the importance of carrier recombination processes.

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Fig. 5. Photocurrent (PC) intensity as a function of applied bias voltage for five SL samples with different barrier thickness (LB ) ranging from 0.57 to 5.17 nm. The PC intensity for each sample is evaluated at wavelengths where the field-induced changes of electroabsorption can be neglected (sample [1: 763 nm, [2: 745 nm, [3: 760 nm, [4: 738 nm, [5: 735 nm).

In order to quantitatively show variations of the PC intensity with the bias, the PC values are plotted in Fig. 5 for the five SL samples. For this plotting, we choose wavelengths for each SL where the electroabsorption features do not significantly influence the PC intensity. Namely, these are 763 nm ([1), 745 nm ([2), 760 nm ([3), 738 nm ([4) and 735 nm ([5). For samples [1 and [3 each wavelength is indicated in Figs. 1 and 3 by vertical dashed lines. However, we note that the dominant features of the PC intensity changes with the bias are invariable with respect to the wavelength selection. In general, the following trends can be seen from Fig. 5. For the thin LB samples whose LB values are less than 6 ML (sample [1 | [3), the PC intensity shows an initially steep increase with the bias (Vb . 0.0 V) and then saturates at nearly the same intensity level. Therefore, the bias (field) dependence of PC increases shows the same behavior regardless of LB for the thin barrier samples. For thick barrier samples ([4 and [5), the PC intensity is strongly dependent on the LB values, very gradually increases and reaches the saturation level under extremely high field conditions. The further increase of PC intensity in the range of 2 25 | 2 30 V is explained by sequential resonant tunneling [9] due to the resonance between the lowest state and the first excited state in the nearest neighbor well. Since photogenerated carriers must reach the contact layer by tunneling to contribute to the photocurrent signal, the tunneling escape time needs to be short enough compared to the recombination times. When the sample is illuminated from the p-side, the photogenerated carriers are generated more in the SL region near the p-contact. Since the number of resonantly absorbed photons decrease as a function of the distance from the front edge of the SL layer they gradually decrease toward the n-contact. Holes can reach the p-contact more easily because most of them are generated near the p-contact. However, electrons must tunnel many barriers to reach the n-contact. Hence the electron transport plays an important role for the photogenerated carriers escape from the intrinsic SL layer.

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The electron tunneling escape times through the AlAs single barrier are estimated, according to our recent study [10], to be 0.2 ps ([1, LB 5 0.57 nm), 2.0 ps ([2, LB 5 1.13 nm), 13.0 ps ([3, LB 5 1.70 nm), 8.0 ns ([4, LB 5 3.42 nm) and 5.0 ms ([5, LB 5 5.17 nm), respectively. Since the recombination lifetimes of GaAs quantum wells are in general of the order of 1 ns, the tunneling escape times for the thin LB samples ([1 | [3) are much shorter than the recombination lifetimes. Hence, in these samples the electrons can escape very easily and contribute to the PC signal before vanishing via recombination processes. On the other hand, for the thicker LB samples ([4 and [5), the tunneling escape times are much longer than the recombination lifetimes. Therefore, the electrons generated in the SL layers efficiently recombine before they reach the contact layer. Hence, under the low field conditions the PC intensity gradually increases with the electric field because most of the electrons are consumed by recombinations. In fact, our recent photoluminescence experiments indicate the expected strong dependence on the barrier thickness of the radiative recombination efficiency. Thus, these results are well explained by assuming that the electron tunneling transport is important for the PC intensity variations with the field. Finally we note that our assumption is consistent with the PC ´ transport mechanisms reported by F. Agullo-Rueda et al. [11]. In summary, we have experimentally investigated photocurrent spectral intensity in GaAs /AlAs superlattices with different barrier thickness (LB 5 2 | 18 monolayers). For thin LB samples, clear evidence is obtained for the Wannier–Stark localization. It is found that the absolute PC intensity changes with the field strength are strongly dependent on the barrier thickness. The observed results are well explained by assuming that the electron tunneling transport is important for the photocurrent intensity and by considering the competition between the recombination lifetime and the tunneling escape time, which is strongly varied by changing the barrier thickness. Acknowledgements The authors would like to thank T. Yamamoto, K. Tominaga and M. Hosoda for their help in the sample preparation at ATR Optical and Radio Communication Research Laboratories. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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