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GaAs MQW PIN diodes
M~roe&ctronicsJournal28(1997) 757-765 O1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 0026-2692/97.,$17.00 ELSEVIER
PII:S0026-2692(96)00114-0
Memory effects on piezoelectric InGaAs/GaAs MQW PIN diodes J.F. Valtuefia~, I. Izpura~, J.L. SanchezRojas 1 E. Mufioz 1 E.A. Khoo 2 J.P.R. David 2, J. Woodhead 2, R. Grey 2 and G.J. Rees 2 It
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tDpto. hlgenieria Eh'ctrdnica, E.T.S.I. Telecomunicacidn, Ciudad Universitaria s/n, 28040 Madrid, Spain 2Departme,~t of Electronic and Electrical Engeneerin¢, University of She~eld, Mappin St, Sheffield S I 3.]13, UK
Chargc accumulation effects in piezoelectric multiple quantum well (MQW) InGaAs/GaAs PIN diodes grown on (111)B GaAs substrates have been studied regarding memory applications. Strain-induced piezoelectric fields allow new PIN structure~; with configurations of negative average electric field (NAF) active region. These new devices can store an electric dipole with spatially separated electrons and holes that have low recombination probability and thus long lifetimes. This produces a longrange screening of the field in the active region and hence a strong blue shift of the absorption band edge (maximum light transmission for reading purposes). Both a light pulse and a forward voltage pulse are able to create the dipole (data writing or charged device). The stored dipole can be removed by a reverse electrical pulse (data erasing or device discharge), resulting in a minimum light transmission across the device. Capacitance voltage and time resolved capacitan:e measurements, after single optical or electrical charging pulse at low temperature (20K) have bccn used to determine the stored dipole behaviour. Capacitance transients analysis allowed study of thc kinetics of the di:/chargc process, which shows a non-exponential bchaviour with storage times up to 103see, suggcsting very long time refresh cycles. Time
resolved photocurrent has been used to check read and write capabilities giving on-off ratios up to 30. © 1997 Elsevier Science Ltd.
1. Introduction he piezoelectric fields induced by strain in structures g r o w n o n polar surfaces have received considerable attention in recent years [1-5]. Piezoelectric fields can be used as a n e w design p a r a m e t e r in o r d e r to i m p r o v e the p e r f o r m a n c e o f electrical and optical devices [2-4]. H i g h optical n o n - l i n e a r ities have b e e n predicted to be associated to an in-well screening [1, 5].
T GaAs/InGaAs
In P I N diodes with an e m b e d d e d M Q W region, these piezoelectric fields allow t w o device configurations w i t h different electro-optical properties. T h e difference is due to the value o f
757
J.F. Valtue a et al./Memory effects
the average electric field (AEF) in the active region: positive average electric field (PAF) or negative average electric field (NAF). Growth parameters such as the indium content, well and barrier width, number of QWs and the total length of the intrinsic region, can be used to design both the sign and magnitude of the AEF [6, 7]. The internal potential band minima at the extreme QWs in NAF structures strongly modifies the behaviour of the device. Photogenerated carriers are swept by the NAF, giving rise to an internal dipole which in turn produces a long-range screening potential on the MQW. The charge accumulation dominates the band profile in the active region and hence the optical and electrical responses as well as the dynamics of photogenerated carriers [8-10]. One remarkable feature associated with the screening is the inhibition of the quantum confined Stark effect (QCSE) under continuous wave (CW) illumination, since the band profile reaches a stationary state independent of the applied voltage [11, 12]. Photoluminescence and photocurrent peaks become clamped, so there exists a blue shift between the observed absorption band edge and the predicted one for the discharged state. The optical non-linearities in such structures are associated with out-ofwell screening [8-13]. These have been demonstrated to be of the same order of magnitude as exciton bleaching and red shift non-linearities induced on (001) substrates, not one order of magnitude higher as previously expected for inwell screening [1-8]. Concerning the long-range screening, the decay kinetics of the created dipole shows long lifetimes due to the spatial separation of electrons and holes [9, 10]. The long recovery time associated with out-of-well screening has been considered to be a drawback for switching applications [8]. However, this slow recombination rate allows the device to work as an electrooptic memory cell.
758
In the present work we have investigated the dynamics of the dipole which produces the non-linear optical response of the device. The study of the electrical and optical charging process, the bias dependence of the maximum obtained screening as well as the discharge process, are focused to demonstrate the electrooptical capabilities of these new devices. 2. Experimental To study the above effects we have designed and grown by molecular beam epitaxy a PIN structure on an n ÷ (111)B GaAs substrate consisting of 14 unintentionally doped In0.1~Ga0.ssAs QWs, 100A wide, separated by 150 A GaAs barriers, grown in the centre of an intrinsic region of 0.85/lm total width. The ntype contact region is formed by 0.3sim of GaAs doped with Si (1018cm-3), and the ptype region is a 0.3/lm GaAs Cap layer doped with Be (1018cm-3). The sample was mesa etched to obtain isolated diodes with an annular top contact to allow optical access, and they were encapsulated on a TO-5 holder. The sample was designed to present NAF configuration in a broad range of bias. The width of the active region (350 nm) was selected to decrease the recombination probability of the stored dipole. Capacitance techniques used for characterization have been shown to be very sensitive to the carrier accumulation in the active region [12, 14]. Such charge accumulation has previously been observed to lead to an increase in device capacitance under C W illumination, but it is difficult to obtain quantitative data in this situation due to the complexity related to the determination of the stationary state under illumination. In order to avoid such a complexity we have investigated the capacitance behaviour in the dark under metastable charge conditions. To decrease thermionic and tunnel escape probabilities the temperature was kept at 20 K by putting the devices in a closed cycle He
Microelectronics Journal, Vol. 28, Nos 8-10
cryostat. Capacitance was monitored by a 7200 Boonton bridge. A time resolved photocurrent was applied to show the read and writing capabilities of this new device. A pulsed laser diode (911nm) producing 7nsec full-width half-maximum pulses was used as the light source for writing action. The transient measurements were performed using a system based on a Tektronix 7854 sampling scope, and the photocurrent was derived from the voltage drop in the 5 0 ~ input resistance of an $4 sampling bead. 3. Results
The transition between NAF and PAF configurations has been determined from time resolved photocurrent [9, 10] and capacitance voltage characteristics under illumination [12, 14], obtaining a bias voltage o f - 1 . 3 5 V . This zero average field (ZAF) condition is defined for zero stored charge (no dipole exists). Figure la shows the C - V characteristics corresponding to the charged and uncharged device. All the C - V characteristics have been obtained in dark as explained previously. The C - V curve corresponding to the discharged device (no stored dipole) was obtained by sweeping voltages from reverse to forward bias. For bias voltages lower than the condition of (ZAF), any internal carriers will flow to the contacts, avoiding any accumulation of charge inside the structure. This C - V curve shows a nearly flat characteristic with no features associated with the transition from PAF to NAF. Only at forward bias higher than 2 V (not shown in the figure due to its high value) the capacitance sharply increases due to the carrier injection driven by forward current. This carrier injection method has been used to populate the internal dipole. This novel charging method presents the advantage of being used as an electrical refreshing method for the memory device. The
C - V curve of the charged device has been obtained by sweeping voltage from forward to reverse bias. It was checked that the carriers were only injected for bias voltages higher than 2.0V. However, it was observed that these remain stored until bias voltage reaches the value of the transition from NAF to PAF, where both C - V curves become the same. The C - V characteristic for the charged state does not depend on the method used to charge the internal dipole. This point has been checked by also using a single light pulse as the charge procedure.
As we will discuss below, it is difficult to extract direct information about the screening potential from the charged device C - V characteristic. Nevertheless, this can be obtained by charging the dipole by a single optical pulse and observing the capacitance-voltage discharge process by means of C - V scanning. This is done by sweeping bias voltage back from +1.8V to a lower and lower final voltage as depicted in Fig. lb. The C - V characteristics obtained in such a way are shown in this figure. The different curves correspond to different states of charge of the dipole, and in each one it is possible to observe three different regions. For bias voltages well above the maximum, the C - V shows a constant voltage shift towards the left (see Fig. l b) from the C - V one corresponding to the discharged structure. As bias is decreased, capacitance increases, reaching a maximum, giving the same value as obtained for the charged device, in spite of the voltage scanning and time spent in the C - V scanning at higher bias voltages. Beyond this point, if bias is decreased the capacitance follows the charged device curve of Fig. la. The C - V characteristics shown in Fig. lb can be explained by taking into account the voltage drop due to the internal stored dipole. The capacitance carries the information about behaviour of the charge at the doped regions Qdop,
759
J.F. Valtue a et al./Memory effects
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Fig. I. (a) The C - V characteristics for the charged and uncharged states. Associated to the charged state, a capacitance increment for bias voltages higher than -1.35V appears, corresponding with the ZAF transition. (b) The C - V characteristics after a single optical charge pulse for voltages swept from +1.8 to increasing reverse voltages, one after the other, to control the discharge process. In order to clarify the figure only one of each of the two sweepings is shown. Three regions are distinguished for each curve, corresponding to the fixed dipole, the reacting dipole and the discharge process. because there is no direct electrical access to the internal charge stored in the Q W s . H o w e v e r , the charges at the d o p e d regions are directly related with the existing screening potential through the Poisson equation. Equation (1) is a simplified form o f it, but can illustrate the process Vui - Vext + nwp - Vsc = Edopi
760
(l)
Where Vbi is the built-in field, V~.x~is the external bias voltage, n is the n u m b e r o f Q W s with a well width o f Lw and barrier Lb, Ep is the piezoelectric field in the strained quantum wells, V~c is the screening voltage produced by the internal dipole, L i is the total length o f the intrinsic region and Eaop is the electric field supported by the doped regions, which is direcdy related with the charge reacting at the contacts.
Microelectronics Journal, Vol. 28, Nos 8-10
It should be noticed that the voltage for the ZAF condition is modified by the screened potential as shown in eq. (2) VcxtiZ^~)(charged) -- Vex~(Z^F)(uncharged) +V~c*( l-nw+(n-1)b)Li
(2)
The derivative of the charge at the doped regions Qdop versus Ve×t, eq. (3), shows all the possible contributions to the capacitance value (C), A being the device area. This expression makes it possible to follow tile changes in the internal screening potential ~c through the changes in the capacitance characteristics. C = d°--Ap= A 1 ext t3s~0Li < Vext+ Vsc > 1 ~c + - -A Ssg0Li <." Vcxt + Vsc > ext
(3)
The presence of a constant voltage shift indicates that the second term of eq. (3), related with the changes of Vsc induced by the external bias voltage (Vext), can be neglected. The dipole is entirely confined at the band minima of the extreme QWs, therefore V~c is kept at a constant value. Then the capacitance for a given external voltage is equal to the uncharged capacitance for an effective voltage V,,,+V~o The constant voltage shift between the uncharged device curve an each curve in Fig. lb corresponds to the term of the screening voltage V~c. When we approach the ZAF, taking into account the internal dipole, an increment in the capacitance value is observed. It is due to the spreading of the charge distributions (eand h ~) inside the active region, modifying the screening potential. When this happens, the second term of eq. (3) takes increasing relevance of the capacitance value. Going towards lower voltage the internal dipole begins to be discharged by extraction and recombination. Then the capacitance fits the C - V character-
istic obtained by sweeping voltages from forward to reverse (Fig. l a). When a new curve is obtained the value of the stored charge is lower than for the previous one, and the voltage shift from the discharge curve is reduced in region 1. The bias voltage for ZAF, a function of the accumulated charge, decreases as Vsc does, then the maximum screening voltage for a given bias voltage is limited to that which satisfies eq. (2). The storage time of the induced internal dipole is the most important feature for memory applications, since it drives the absorption changes by means of QCSE. Time resolved capacitance after charging has been used to determine the time evolution of the screening voltage. Carrier injection is produced by a voltage pulse of +2 V, as discussed above. The capacitance transients are shown in Fig. 2 as a function of the bias voltage. For negative time values the stationary value of the capacitance for the discharged state, prior to the application of the voltage charging pulse, is presented as a reference. The initial transients show a clear non-exponential behaviour associated with the discharge process. After the injection, the capacitance value corresponds to that obtained for the charged device in Fig. la. It follows a decay of the order of several seconds before reaching a nearly constant value. This behaviour can be explained as follows: during the first seconds the device is near the ZAF condition, having maximum overlapping between electron and hole wavefunctions. As soon as the dipole begins to recombine, the screening potential for a given voltage decreases; increasing the confinement of the remaining charge, this process gives rise to a decreasing recombination rate during the discharge process. Once most of the charge is well confined at the extreme QWs, spatially separated by more than 300 nm, the recombination is strongly reduced. Therefore, the capacitance transients show tails corresponding to extremely large time constants (=10 3 see), and
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Time (s) Fig. 2. Capacitance transients after a light pulse for different bias voltages. Capacitance corresponding to the uncharged state is presented as reference at negative time values. They are observed as clear non-exponential transients in all the range of bias.
the device appears to be in a metastable charged state. The optical pump electrical probe technique performed at room temperature gives sample discharge time constants exceeding milliseconds. The charge state of the device will define its optical behaviour. Photocurrent spectrum as a function of the applied voltage has been used to estimate the absorption blue shift accomplished by the structure in the charged state. Figure 3a shows these photocurrent spectra under C W illumination taken at low illumination power (0.1 #W/cm2). The spectra show a blue shift for increasing reverse voltages; the blue shift exists until ZAF is reached (V~xt=-l.35V). Above this bias voltage the charge accumulated by continuous illumination maintains the band structure in the ZAF condition, and the absorption band edge becomes fixed. Then the EE1HH1 exciton is blue shifted from the one calculated for a discharged device, Fig. 3b. Maximum blue shift between charged and uncharged states is obtained at the highest forward bias. For wavelengths higher than 902 nm, the absorption differences can be used
762
The different values of the absorption coefficient for the O N and OFF states have been measured by time resolved photocurrent. The wavelength of the pulsed laser (911 nm) was selected to reach maximum absorption at the O N state and minimum at the OFF state; a shift of ~11 nm is expected for a bias voltage of +1 V (Fig. 3). To obtain the OFF state (uncharged) a - 5 V voltage pulse of 30 nsec width was superimposed on the dc bias voltage just before each optical pulse. The voltage pulse sweeps out all the remaining charge accumulated from the previous light pulse, leaving the device at the OFF state (erasing procedure). It was observed that the photocurrent transients were independent of the time position of the bias voltage pulse with respect to the optical pulse, as expected for a stationary OFF state. Figure 4 shows photocurrent transients for different bias voltages. Photocurrent amplitude is suppressed for bias lower than -0.4V, and increases at forward bias; this behaviour is in agreement with the simulated values of the absorption band edge for the OFF (uncharged) state, Fig. 3b. For the OFF state the amplitude of the photocurrent transient is proportional to the absorption coefficient at the laser wavelength. For a bias voltage of +1 V a maximum screening voltage of +2V was observed (Fig. lb), higher than the voltage shift necessary to quench the absorption (1.4V). For the O N state a strong decrease of the photocurrent is expected at this bias. The same continuous train of pulses without the erasing voltage pulse was employed to provide the O N state. The repetition rate was fixed to 250Hz, the cycles being faster than the observed discharge process (Fig. 2). Then the dipole was charged, depending on the laser power, by a few optical pulses. Photocurrent at the O N state is strongly suppressed, as shown
Microelectronics Journal, Vol. 28, Nos 8-10
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Taking advantage o f the previous analysis a new device for memory applications is proposed, It shows two states for light transmission ( O N and OFF) which are a function o f thc charge statc of the device. The device optical writing and reading, long time storage, reverse biasing for fast erasing, and the electrical refreshment by forward pulse biasing have been tested. Transient photocurrent ratios higher than 30 have been reported between O N and O F F states. The results obtained are encouraging considering that it is a non-optimized structure.
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Acknowledgments by dots in Fig. 4 for a bias voltage o f ~1 V. It should be noticed that for the O N response no differences with bias voltages have been observed. The photocurrent in thc O N state is strongly reduced due to the blue shift induced by the screening voltage ~ 2 V, providing photocurrent O N - O F F ratios up to 30.
4. Conclusions The storage bchaviour o f M Q W PIN diodes under N A F conditions has bcen studied. The ZAF condition governs the maxinmm scrccning voltage as observcd for C W wave illumination. A simple analytical exprcssion l%r the screening voltage as a function o f the extcmal bias vohagc has been proposed. This expression has been used to explain the capacitance behaviour, and to obtain the maximum screening vohagc for a given bias. Storage times up to 103 sec. have been observed at low temperature (20 K), with a clearly nonexponential behaviour associated with the electrical confinement o f carriers at the extreme QWs. The linear optical response has been
764
The financial support o f the C I C Y T project TIC95-0116 and the Spanish program Acciones Integradas, action number HB94-093, are acknowledged.
References [1] Smith, D.L. and Mailhiot, C. Optical properties of strained-layer superlattices with growth axis along [111]. Phys. Rev. l_x'tt., 58 (1987) 1264. [20 Sela, I., Watkins, D.E., Laurich, B.K., Smith, l)i., Subbanna, S. and Krocmer, H. Excitonic optical nonlineari~" induced by internal field screening in (211) oriented strained-layer superlattices. Appl. Phys. Lett., 58(7) (1991) 684. [3] Goossen, K.W., Caridi, E.A., Chang, T.Y., Stark, J.B., Miller, D.A.B. and Morgan, R..A. Observation of room temperature blue shift and bistability m a strained lnGaAs-GaAs <111> self-electrooptic effect device. Appl. Phys. Lett., 56(8) (1989) 715. 14] Hcrnandez, J.M., Izpura, 1., CalleD , E. and Mufioz, E. Piezoelectric-induced current asymmetry in 111} InGaAs/lnAIAs resonant tunnelling diodes for microwave mixing. Appl. Phys. Leu., 63 (1993) 773. [51 Moise, T.S., Guido, LJ., Barker, R..C., White, J.O. and Kost, A.I'Z. Screening effects in (111)B AIGaAslnGaAs single quantum well heterostructures. Appl. Phys. Lett., 60(21) (1992) 2637.
Microelectronics Journal, Vol. 28, Nos 8-10
[6] Pabla, A.S., Sanche:'.-P, ojas, J.L., Woodhead,J., (;rey, R., David, J.P.P,., Rees, G.J., Hill, G., Pate, M.A., Robson, P.N., Hog,g, R.A., Fishcr, T.A., Willcox, R.K., Whittaker, D.M. and Skolnick, M.S. Tailoring of internal fields in InGaAs/GaAs multiple quantum well structures grown on (II1)B GaAs. Appl. Phys. l_x'tt., 63(6) (1993)"752. [7] Sanchez-Rojas, J.L., Saccd6n, A., Callc, F., Calleja, E. and Mt, fioz, E. Conduction-band engineering in piezoelectric [111] multiple quantum well p-i-n photodiodes. Appl. Phys. Lett., 65(17) (1994) 2214. [8] Cartwright, A.N., McCallum, D.S., Boggess, T.F., Smirl, A.L., Moist. T.S., Guido, k.J., Barker, R.C. and Wherrett, B.S. Magnitude, origin and evolution of piezoelectric optical nonlinearities m strained 11 lIB InGaAs/GaAs quantum wells. J. Appl. Phys., 73(11) (1993) 7767 [9] Valtuefia, J.F., Izpura, I., Sanchez-lkojas, J.L., Saced6n, A., Calleja, E. and Mufioz, E. l)isplacemcnt photocurrcnts and screening cffects in novcl piezoelectric [nGaAs/GaAs multiple-quantum-well PIN diodes. Semicond. Sc.:. Technol., 10 (1995) 1528. [10] Izpt, ra, 1., Vahuefia, J.F., Sfinchez-Rojas, J.L., Saced6n, A. and Mufioz, E. Transient negativc photo-
[11]
[12]
[13]
[14]
current and out of well dipole kinetics in piezoelectric MQW PIN diodes. Solid State lilectron., 40 (1996) 463--467. Sanchez-R.ojas, J i . , Saced6n, A., Callc, F., Calleja, E. and Mufioz, E. Conduction band engineering in InGaAs/GaAs [111] muhiple quantum well pin photodiodes. Superlatt. Microstmct., 14(3) (1994) 287. Sanchez-P,ojas, J.L., Mufioz, E., Saced6n, A., Valtuefia, J.F., Izpura I. and Calleja, E. Field control in piezoelectric Ill 1l-oriented lnGaAs/GaAs MQW and superlattice devices, Proc. ('onf on Advanced Concepts in High Stx,ed Semiconductor l)evices and Circuits, Cornell, 1995, p. 506 Huang, X.R., Harken, D.R., Cartwright, A.N., McCallum, I).S., Smirl, A.L., S;inchez-IKojas, J.L., Saced6n, A., Gonzfilez-Sanz, F., Calkja, E. and Mufioz, E. Non linear optical response, screening, and distribution of strain in piezoelectric multiple quantmn wclls.J. Appl. Phys., 76(12) (1994) 787(I. Sanchcz-Rojas, J.L., Saced6n, A., Calleja, E., Mu~oz, E., Sanz-Herv;is, A., DeBenito, G. and L6pez, M. Photoinibition of the quantum confined Stark effect in piezoelectric muhiple quantum wells. Phys. Rev. B, 53(23) (1996) 15469.
765
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Memory effects on piezoelectric InGaAs/GaAs MQW PIN diodes J.F. Valtuefia~, I. Izpura~, J.L. SanchezRojas 1 E. Mufioz 1 E.A. Khoo 2 J.P.R. David 2, J. Woodhead 2, R. Grey 2 and G.J. Rees 2 It
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tDpto. hlgenieria Eh'ctrdnica, E.T.S.I. Telecomunicacidn, Ciudad Universitaria s/n, 28040 Madrid, Spain 2Departme,~t of Electronic and Electrical Engeneerin¢, University of She~eld, Mappin St, Sheffield S I 3.]13, UK
Chargc accumulation effects in piezoelectric multiple quantum well (MQW) InGaAs/GaAs PIN diodes grown on (111)B GaAs substrates have been studied regarding memory applications. Strain-induced piezoelectric fields allow new PIN structure~; with configurations of negative average electric field (NAF) active region. These new devices can store an electric dipole with spatially separated electrons and holes that have low recombination probability and thus long lifetimes. This produces a longrange screening of the field in the active region and hence a strong blue shift of the absorption band edge (maximum light transmission for reading purposes). Both a light pulse and a forward voltage pulse are able to create the dipole (data writing or charged device). The stored dipole can be removed by a reverse electrical pulse (data erasing or device discharge), resulting in a minimum light transmission across the device. Capacitance voltage and time resolved capacitan:e measurements, after single optical or electrical charging pulse at low temperature (20K) have bccn used to determine the stored dipole behaviour. Capacitance transients analysis allowed study of thc kinetics of the di:/chargc process, which shows a non-exponential bchaviour with storage times up to 103see, suggcsting very long time refresh cycles. Time
resolved photocurrent has been used to check read and write capabilities giving on-off ratios up to 30. © 1997 Elsevier Science Ltd.
1. Introduction he piezoelectric fields induced by strain in structures g r o w n o n polar surfaces have received considerable attention in recent years [1-5]. Piezoelectric fields can be used as a n e w design p a r a m e t e r in o r d e r to i m p r o v e the p e r f o r m a n c e o f electrical and optical devices [2-4]. H i g h optical n o n - l i n e a r ities have b e e n predicted to be associated to an in-well screening [1, 5].
T GaAs/InGaAs
In P I N diodes with an e m b e d d e d M Q W region, these piezoelectric fields allow t w o device configurations w i t h different electro-optical properties. T h e difference is due to the value o f
757
J.F. Valtue a et al./Memory effects
the average electric field (AEF) in the active region: positive average electric field (PAF) or negative average electric field (NAF). Growth parameters such as the indium content, well and barrier width, number of QWs and the total length of the intrinsic region, can be used to design both the sign and magnitude of the AEF [6, 7]. The internal potential band minima at the extreme QWs in NAF structures strongly modifies the behaviour of the device. Photogenerated carriers are swept by the NAF, giving rise to an internal dipole which in turn produces a long-range screening potential on the MQW. The charge accumulation dominates the band profile in the active region and hence the optical and electrical responses as well as the dynamics of photogenerated carriers [8-10]. One remarkable feature associated with the screening is the inhibition of the quantum confined Stark effect (QCSE) under continuous wave (CW) illumination, since the band profile reaches a stationary state independent of the applied voltage [11, 12]. Photoluminescence and photocurrent peaks become clamped, so there exists a blue shift between the observed absorption band edge and the predicted one for the discharged state. The optical non-linearities in such structures are associated with out-ofwell screening [8-13]. These have been demonstrated to be of the same order of magnitude as exciton bleaching and red shift non-linearities induced on (001) substrates, not one order of magnitude higher as previously expected for inwell screening [1-8]. Concerning the long-range screening, the decay kinetics of the created dipole shows long lifetimes due to the spatial separation of electrons and holes [9, 10]. The long recovery time associated with out-of-well screening has been considered to be a drawback for switching applications [8]. However, this slow recombination rate allows the device to work as an electrooptic memory cell.
758
In the present work we have investigated the dynamics of the dipole which produces the non-linear optical response of the device. The study of the electrical and optical charging process, the bias dependence of the maximum obtained screening as well as the discharge process, are focused to demonstrate the electrooptical capabilities of these new devices. 2. Experimental To study the above effects we have designed and grown by molecular beam epitaxy a PIN structure on an n ÷ (111)B GaAs substrate consisting of 14 unintentionally doped In0.1~Ga0.ssAs QWs, 100A wide, separated by 150 A GaAs barriers, grown in the centre of an intrinsic region of 0.85/lm total width. The ntype contact region is formed by 0.3sim of GaAs doped with Si (1018cm-3), and the ptype region is a 0.3/lm GaAs Cap layer doped with Be (1018cm-3). The sample was mesa etched to obtain isolated diodes with an annular top contact to allow optical access, and they were encapsulated on a TO-5 holder. The sample was designed to present NAF configuration in a broad range of bias. The width of the active region (350 nm) was selected to decrease the recombination probability of the stored dipole. Capacitance techniques used for characterization have been shown to be very sensitive to the carrier accumulation in the active region [12, 14]. Such charge accumulation has previously been observed to lead to an increase in device capacitance under C W illumination, but it is difficult to obtain quantitative data in this situation due to the complexity related to the determination of the stationary state under illumination. In order to avoid such a complexity we have investigated the capacitance behaviour in the dark under metastable charge conditions. To decrease thermionic and tunnel escape probabilities the temperature was kept at 20 K by putting the devices in a closed cycle He
Microelectronics Journal, Vol. 28, Nos 8-10
cryostat. Capacitance was monitored by a 7200 Boonton bridge. A time resolved photocurrent was applied to show the read and writing capabilities of this new device. A pulsed laser diode (911nm) producing 7nsec full-width half-maximum pulses was used as the light source for writing action. The transient measurements were performed using a system based on a Tektronix 7854 sampling scope, and the photocurrent was derived from the voltage drop in the 5 0 ~ input resistance of an $4 sampling bead. 3. Results
The transition between NAF and PAF configurations has been determined from time resolved photocurrent [9, 10] and capacitance voltage characteristics under illumination [12, 14], obtaining a bias voltage o f - 1 . 3 5 V . This zero average field (ZAF) condition is defined for zero stored charge (no dipole exists). Figure la shows the C - V characteristics corresponding to the charged and uncharged device. All the C - V characteristics have been obtained in dark as explained previously. The C - V curve corresponding to the discharged device (no stored dipole) was obtained by sweeping voltages from reverse to forward bias. For bias voltages lower than the condition of (ZAF), any internal carriers will flow to the contacts, avoiding any accumulation of charge inside the structure. This C - V curve shows a nearly flat characteristic with no features associated with the transition from PAF to NAF. Only at forward bias higher than 2 V (not shown in the figure due to its high value) the capacitance sharply increases due to the carrier injection driven by forward current. This carrier injection method has been used to populate the internal dipole. This novel charging method presents the advantage of being used as an electrical refreshing method for the memory device. The
C - V curve of the charged device has been obtained by sweeping voltage from forward to reverse bias. It was checked that the carriers were only injected for bias voltages higher than 2.0V. However, it was observed that these remain stored until bias voltage reaches the value of the transition from NAF to PAF, where both C - V curves become the same. The C - V characteristic for the charged state does not depend on the method used to charge the internal dipole. This point has been checked by also using a single light pulse as the charge procedure.
As we will discuss below, it is difficult to extract direct information about the screening potential from the charged device C - V characteristic. Nevertheless, this can be obtained by charging the dipole by a single optical pulse and observing the capacitance-voltage discharge process by means of C - V scanning. This is done by sweeping bias voltage back from +1.8V to a lower and lower final voltage as depicted in Fig. lb. The C - V characteristics obtained in such a way are shown in this figure. The different curves correspond to different states of charge of the dipole, and in each one it is possible to observe three different regions. For bias voltages well above the maximum, the C - V shows a constant voltage shift towards the left (see Fig. l b) from the C - V one corresponding to the discharged structure. As bias is decreased, capacitance increases, reaching a maximum, giving the same value as obtained for the charged device, in spite of the voltage scanning and time spent in the C - V scanning at higher bias voltages. Beyond this point, if bias is decreased the capacitance follows the charged device curve of Fig. la. The C - V characteristics shown in Fig. lb can be explained by taking into account the voltage drop due to the internal stored dipole. The capacitance carries the information about behaviour of the charge at the doped regions Qdop,
759
J.F. Valtue a et al./Memory effects
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Fig. I. (a) The C - V characteristics for the charged and uncharged states. Associated to the charged state, a capacitance increment for bias voltages higher than -1.35V appears, corresponding with the ZAF transition. (b) The C - V characteristics after a single optical charge pulse for voltages swept from +1.8 to increasing reverse voltages, one after the other, to control the discharge process. In order to clarify the figure only one of each of the two sweepings is shown. Three regions are distinguished for each curve, corresponding to the fixed dipole, the reacting dipole and the discharge process. because there is no direct electrical access to the internal charge stored in the Q W s . H o w e v e r , the charges at the d o p e d regions are directly related with the existing screening potential through the Poisson equation. Equation (1) is a simplified form o f it, but can illustrate the process Vui - Vext + nwp - Vsc = Edopi
760
(l)
Where Vbi is the built-in field, V~.x~is the external bias voltage, n is the n u m b e r o f Q W s with a well width o f Lw and barrier Lb, Ep is the piezoelectric field in the strained quantum wells, V~c is the screening voltage produced by the internal dipole, L i is the total length o f the intrinsic region and Eaop is the electric field supported by the doped regions, which is direcdy related with the charge reacting at the contacts.
Microelectronics Journal, Vol. 28, Nos 8-10
It should be noticed that the voltage for the ZAF condition is modified by the screened potential as shown in eq. (2) VcxtiZ^~)(charged) -- Vex~(Z^F)(uncharged) +V~c*( l-nw+(n-1)b)Li
(2)
The derivative of the charge at the doped regions Qdop versus Ve×t, eq. (3), shows all the possible contributions to the capacitance value (C), A being the device area. This expression makes it possible to follow tile changes in the internal screening potential ~c through the changes in the capacitance characteristics. C = d°--Ap= A 1 ext t3s~0Li < Vext+ Vsc > 1 ~c + - -A Ssg0Li <." Vcxt + Vsc > ext
(3)
The presence of a constant voltage shift indicates that the second term of eq. (3), related with the changes of Vsc induced by the external bias voltage (Vext), can be neglected. The dipole is entirely confined at the band minima of the extreme QWs, therefore V~c is kept at a constant value. Then the capacitance for a given external voltage is equal to the uncharged capacitance for an effective voltage V,,,+V~o The constant voltage shift between the uncharged device curve an each curve in Fig. lb corresponds to the term of the screening voltage V~c. When we approach the ZAF, taking into account the internal dipole, an increment in the capacitance value is observed. It is due to the spreading of the charge distributions (eand h ~) inside the active region, modifying the screening potential. When this happens, the second term of eq. (3) takes increasing relevance of the capacitance value. Going towards lower voltage the internal dipole begins to be discharged by extraction and recombination. Then the capacitance fits the C - V character-
istic obtained by sweeping voltages from forward to reverse (Fig. l a). When a new curve is obtained the value of the stored charge is lower than for the previous one, and the voltage shift from the discharge curve is reduced in region 1. The bias voltage for ZAF, a function of the accumulated charge, decreases as Vsc does, then the maximum screening voltage for a given bias voltage is limited to that which satisfies eq. (2). The storage time of the induced internal dipole is the most important feature for memory applications, since it drives the absorption changes by means of QCSE. Time resolved capacitance after charging has been used to determine the time evolution of the screening voltage. Carrier injection is produced by a voltage pulse of +2 V, as discussed above. The capacitance transients are shown in Fig. 2 as a function of the bias voltage. For negative time values the stationary value of the capacitance for the discharged state, prior to the application of the voltage charging pulse, is presented as a reference. The initial transients show a clear non-exponential behaviour associated with the discharge process. After the injection, the capacitance value corresponds to that obtained for the charged device in Fig. la. It follows a decay of the order of several seconds before reaching a nearly constant value. This behaviour can be explained as follows: during the first seconds the device is near the ZAF condition, having maximum overlapping between electron and hole wavefunctions. As soon as the dipole begins to recombine, the screening potential for a given voltage decreases; increasing the confinement of the remaining charge, this process gives rise to a decreasing recombination rate during the discharge process. Once most of the charge is well confined at the extreme QWs, spatially separated by more than 300 nm, the recombination is strongly reduced. Therefore, the capacitance transients show tails corresponding to extremely large time constants (=10 3 see), and
761
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the device appears to be in a metastable charged state. The optical pump electrical probe technique performed at room temperature gives sample discharge time constants exceeding milliseconds. The charge state of the device will define its optical behaviour. Photocurrent spectrum as a function of the applied voltage has been used to estimate the absorption blue shift accomplished by the structure in the charged state. Figure 3a shows these photocurrent spectra under C W illumination taken at low illumination power (0.1 #W/cm2). The spectra show a blue shift for increasing reverse voltages; the blue shift exists until ZAF is reached (V~xt=-l.35V). Above this bias voltage the charge accumulated by continuous illumination maintains the band structure in the ZAF condition, and the absorption band edge becomes fixed. Then the EE1HH1 exciton is blue shifted from the one calculated for a discharged device, Fig. 3b. Maximum blue shift between charged and uncharged states is obtained at the highest forward bias. For wavelengths higher than 902 nm, the absorption differences can be used
762
The different values of the absorption coefficient for the O N and OFF states have been measured by time resolved photocurrent. The wavelength of the pulsed laser (911 nm) was selected to reach maximum absorption at the O N state and minimum at the OFF state; a shift of ~11 nm is expected for a bias voltage of +1 V (Fig. 3). To obtain the OFF state (uncharged) a - 5 V voltage pulse of 30 nsec width was superimposed on the dc bias voltage just before each optical pulse. The voltage pulse sweeps out all the remaining charge accumulated from the previous light pulse, leaving the device at the OFF state (erasing procedure). It was observed that the photocurrent transients were independent of the time position of the bias voltage pulse with respect to the optical pulse, as expected for a stationary OFF state. Figure 4 shows photocurrent transients for different bias voltages. Photocurrent amplitude is suppressed for bias lower than -0.4V, and increases at forward bias; this behaviour is in agreement with the simulated values of the absorption band edge for the OFF (uncharged) state, Fig. 3b. For the OFF state the amplitude of the photocurrent transient is proportional to the absorption coefficient at the laser wavelength. For a bias voltage of +1 V a maximum screening voltage of +2V was observed (Fig. lb), higher than the voltage shift necessary to quench the absorption (1.4V). For the O N state a strong decrease of the photocurrent is expected at this bias. The same continuous train of pulses without the erasing voltage pulse was employed to provide the O N state. The repetition rate was fixed to 250Hz, the cycles being faster than the observed discharge process (Fig. 2). Then the dipole was charged, depending on the laser power, by a few optical pulses. Photocurrent at the O N state is strongly suppressed, as shown
Microelectronics Journal, Vol. 28, Nos 8-10
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Taking advantage o f the previous analysis a new device for memory applications is proposed, It shows two states for light transmission ( O N and OFF) which are a function o f thc charge statc of the device. The device optical writing and reading, long time storage, reverse biasing for fast erasing, and the electrical refreshment by forward pulse biasing have been tested. Transient photocurrent ratios higher than 30 have been reported between O N and O F F states. The results obtained are encouraging considering that it is a non-optimized structure.
I,",.~,- ' 1 V.
Acknowledgments by dots in Fig. 4 for a bias voltage o f ~1 V. It should be noticed that for the O N response no differences with bias voltages have been observed. The photocurrent in thc O N state is strongly reduced due to the blue shift induced by the screening voltage ~ 2 V, providing photocurrent O N - O F F ratios up to 30.
4. Conclusions The storage bchaviour o f M Q W PIN diodes under N A F conditions has bcen studied. The ZAF condition governs the maxinmm scrccning voltage as observcd for C W wave illumination. A simple analytical exprcssion l%r the screening voltage as a function o f the extcmal bias vohagc has been proposed. This expression has been used to explain the capacitance behaviour, and to obtain the maximum screening vohagc for a given bias. Storage times up to 103 sec. have been observed at low temperature (20 K), with a clearly nonexponential behaviour associated with the electrical confinement o f carriers at the extreme QWs. The linear optical response has been
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The financial support o f the C I C Y T project TIC95-0116 and the Spanish program Acciones Integradas, action number HB94-093, are acknowledged.
References [1] Smith, D.L. and Mailhiot, C. Optical properties of strained-layer superlattices with growth axis along [111]. Phys. Rev. l_x'tt., 58 (1987) 1264. [20 Sela, I., Watkins, D.E., Laurich, B.K., Smith, l)i., Subbanna, S. and Krocmer, H. Excitonic optical nonlineari~" induced by internal field screening in (211) oriented strained-layer superlattices. Appl. Phys. Lett., 58(7) (1991) 684. [3] Goossen, K.W., Caridi, E.A., Chang, T.Y., Stark, J.B., Miller, D.A.B. and Morgan, R..A. Observation of room temperature blue shift and bistability m a strained lnGaAs-GaAs <111> self-electrooptic effect device. Appl. Phys. Lett., 56(8) (1989) 715. 14] Hcrnandez, J.M., Izpura, 1., CalleD , E. and Mufioz, E. Piezoelectric-induced current asymmetry in 111} InGaAs/lnAIAs resonant tunnelling diodes for microwave mixing. Appl. Phys. Leu., 63 (1993) 773. [51 Moise, T.S., Guido, LJ., Barker, R..C., White, J.O. and Kost, A.I'Z. Screening effects in (111)B AIGaAslnGaAs single quantum well heterostructures. Appl. Phys. Lett., 60(21) (1992) 2637.
Microelectronics Journal, Vol. 28, Nos 8-10
[6] Pabla, A.S., Sanche:'.-P, ojas, J.L., Woodhead,J., (;rey, R., David, J.P.P,., Rees, G.J., Hill, G., Pate, M.A., Robson, P.N., Hog,g, R.A., Fishcr, T.A., Willcox, R.K., Whittaker, D.M. and Skolnick, M.S. Tailoring of internal fields in InGaAs/GaAs multiple quantum well structures grown on (II1)B GaAs. Appl. Phys. l_x'tt., 63(6) (1993)"752. [7] Sanchez-Rojas, J.L., Saccd6n, A., Callc, F., Calleja, E. and Mt, fioz, E. Conduction-band engineering in piezoelectric [111] multiple quantum well p-i-n photodiodes. Appl. Phys. Lett., 65(17) (1994) 2214. [8] Cartwright, A.N., McCallum, D.S., Boggess, T.F., Smirl, A.L., Moist. T.S., Guido, k.J., Barker, R.C. and Wherrett, B.S. Magnitude, origin and evolution of piezoelectric optical nonlinearities m strained 11 lIB InGaAs/GaAs quantum wells. J. Appl. Phys., 73(11) (1993) 7767 [9] Valtuefia, J.F., Izpura, I., Sanchez-lkojas, J.L., Saced6n, A., Calleja, E. and Mufioz, E. l)isplacemcnt photocurrcnts and screening cffects in novcl piezoelectric [nGaAs/GaAs multiple-quantum-well PIN diodes. Semicond. Sc.:. Technol., 10 (1995) 1528. [10] Izpt, ra, 1., Vahuefia, J.F., Sfinchez-Rojas, J.L., Saced6n, A. and Mufioz, E. Transient negativc photo-
[11]
[12]
[13]
[14]
current and out of well dipole kinetics in piezoelectric MQW PIN diodes. Solid State lilectron., 40 (1996) 463--467. Sanchez-R.ojas, J i . , Saced6n, A., Callc, F., Calleja, E. and Mufioz, E. Conduction band engineering in InGaAs/GaAs [111] muhiple quantum well pin photodiodes. Superlatt. Microstmct., 14(3) (1994) 287. Sanchez-P,ojas, J.L., Mufioz, E., Saced6n, A., Valtuefia, J.F., Izpura I. and Calleja, E. Field control in piezoelectric Ill 1l-oriented lnGaAs/GaAs MQW and superlattice devices, Proc. ('onf on Advanced Concepts in High Stx,ed Semiconductor l)evices and Circuits, Cornell, 1995, p. 506 Huang, X.R., Harken, D.R., Cartwright, A.N., McCallum, I).S., Smirl, A.L., S;inchez-IKojas, J.L., Saced6n, A., Gonzfilez-Sanz, F., Calkja, E. and Mufioz, E. Non linear optical response, screening, and distribution of strain in piezoelectric multiple quantmn wclls.J. Appl. Phys., 76(12) (1994) 787(I. Sanchcz-Rojas, J.L., Saced6n, A., Calleja, E., Mu~oz, E., Sanz-Herv;is, A., DeBenito, G. and L6pez, M. Photoinibition of the quantum confined Stark effect in piezoelectric muhiple quantum wells. Phys. Rev. B, 53(23) (1996) 15469.
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