Transient negative photocurrent and out-of-well dipole kinetics in novel piezoelectric InGaAsGaAs MQW pin diodes

Solid-Store Ekrronics Vol. 40, Nos l-8. pp. 463467, 1996 Copyright 0 1996 Elscvier Science Ltd Printed in Great Britain. All rights reserved 003%1101(%)00311-8

Pergamon

0038-tlOI/

$15.00+ 0.00

TRANSIENT NEGATIVE PHOTOCURRENT AND OUT-OF-WELL DIPOLE KINETICS IN NOVEL PIEZOELECTRIC InGaAs/GaAs MQW pin DIODES 1. IZPURA, Departamento

J. F. VALTUERA, J. L. SANCHEZ-ROJAS, A. SACED6N. E. CALLEJA and E. MUROZ de Ingenieria Electronica, ETSI Telecomunicacion, Universidad Politecnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain

Abstract-In (I I I)B InGaAs/GaAs pin structures with a multiple quantum well (MQW) embedded region, the average internal field in the active MQW region can be tailored to obtain device configurations with a negative average field (NAF), opposite to the built-in field. In (I 1I) NAF diodes, carriers photogenerated at the wells become trapped early at the potential minima located at the ends of the active region thus creating an electric dipole. In this work, in (I I 1) NAF devices with a 0. I7 In mole fraction layers, by using time-resolved photocurrent and a novel optical-pump electrical-probe techniques, we report the presence of a negative transient photocurrent, a direct quantitative evidence of such dipole formation, and we present measurements of its extinction kinetics at room temperature.

1. INTRODUCTION Zinc-blende semiconductor strained layers, grown on substrates different from the (100) orientation present an internal piezoelectric field that is maximized for pseudomorphic structures on (I 11) substrates[l].

InGaAs/GaAs (I 1l)B pin structures with a MQW embedded region, have been used to study the optoelectronic properties associated with the piezoelectric fields. It has been shown that the average, or envelope internal field in the MQW active region can be tailored in both sign and magnitude. Comparing the directions of the built-in field and of such average field, two diode configurations have been reported, NAF (negative average field) and PAF (positive average field) devices[2]. As in standard (100) pin photodiodes, in PAF devices under proper illumination, electrons and holes generated in the wells try to escape out by tunneling and thermoionic processes, and they are drifted by the average electric field towards the n+ (electrons) and p+ (holes) regions, respectively. A positive photocurrent is produced. However, in (111) NAF samples, because the envelope field has a negative sign, the carriers generated in the wells are drifted in the reverse senses, i.e. electrons (holes) towards the p + (n + ) contact. But the carriers are trapped early at the potential minima located at the ends of the active region, forming a charge dipole, and producing an out-of-well screening of the internal fields. The above ideas of carrier dipole formation and long range screening effects were used to explain the photoluminescence and time-resolved pump and

probe absorption measurements, at low temperatures, in ( 11I ) NAF structures[2l]. Besides, out-ofwell screening has been proposed as the principal source of optical non-linearities in (111) NAF devices[5,6]. Because of the importance of confirming and clarifying the above charge accumulation and screening mechanisms in NAF structures, a time domain analysis of the photocurrent in these diodes has been addressed in this work. It is directly shown that the drift of photocarriers towards the ends of the MQW active region produces a negative external photocurrent. It is found that the spatial separation of these electrons and holes (dipole) decreases their recombination probability, and it is responsible for carrier recombination times well above the microsecond range.

2. EXPERIMENTAL pin structures have been designed and grown by molecular beam epitaxy on n+ doped (I 1l)B and (100) GaAs substrates, containing 10 undoped In0.,,Ga0,8,As 95 A QWs separated by 145 A GaAs barriers inside an intrinsic region of 0.57pm total width. The n+-type contact region is formed by 0.3 pm of GaAs doped with Si (10’8cm-‘), and the p+-type region is a 0.3 pm GaAs cap layer doped with Be (lO’*cm-‘). Samples were mesa etched to obtain isolated diodes, and a top contact was fabricated in the form of a ring with 1000 and SOOpm external and internal diameters, respectively. The (Ill) structures were designed to have a negative

463

I. Izpura ef al.

464 0.3 1100

x 10

I

urn

GaAs

2 lO’s[Be]

A

GaAs

u .d

145 A 95 A

GaAs InGaAs

1100

A

GaAs

0.3

pm

GaAs

Substrate

GaAs

x=17%

u.d. u.d. u

.d

2 lO’s[Si] n’
Fig. 1. Layer structure of the pin diodes grown on (100) and (111) substrates and band diagram at equilibrium of the (111) samples. In the last ones the average field in the MQW can be tailored in sense and magnitude by proper design and by applying an external bias voltage[2].

average field (NAF) at 0 V, as shown in Fig. 1. Thus the magnitude of the average field will be controlled in magnitude and sign by a d.c. external bias voltage. Transient photocurrent measurements were performed at room temperature in a system based Tektronix 7854 sampling scope. Photocurrent was monitored through the voltage drop in the 50 ohms input resistance of the S4 sampling head. A pulsed laser diode (905 nm) was used to photogenerate carriers just in the QW region, producing optical pulses with 5 ns full width half maximum at a repetition frequency in the l-10 kHz range. The dynamics of the charge dipole fading away was monitored by an optical pulse (pump) and electrical pulse (probe) technique, using a second experimental set-up. In our experiments a negative electrical pulse (-2 V) was superimposed on the d.c. diode bias at controllable delay times after the optical pump pulse. This electrical probe pulse changes the existing negative average field in the diode to a positive one. and the remaining dipole charge is then extracted and measured by the probe pulse. The pulse sequence and the electrical circuit configuration are shown in Fig. 2.

states are obtained depending on the sign of the average electric field (AEF) in the MQW region. These states are the above mentioned positive average field (PAF) and negative average field (NAF) related to the sign of the field in the barrier regions. A very different transport behavior is expected for each state. Figure 3 shows the photocurrent transients as a function of the applied bias voltage for both (100) and (111) samples. In (100) samples the measurements show the well known behavior of a pin diode whose response is limited by the RC time constant due to the load resistance and diode capacitance. Such response becomes faster and more efficient as the internal field is increased by higher reverse biasing. The same behavior is present in the (I 11) sample for bias voltages more negative than -0.7 V, but when the bias voltage is higher than -0.7 V, a new feature is observed at the beginning of the transients: a negative value of the photocurrent whose amplitude increases and dominates the response as the bias voltage is more positive. Taking the value of -0.7 V as the transition from PAF to NAF state, this voltage should produce the zero average field condition. This allows to determine the piezoelectric field: 130 kV cm-’ and the piezoelectric constant: 0.075 C m-’ by using the nominal growth parameters[2,7,8]. These results are in good

Optical Pump k L dela) Voltage

t

)

Probe

3. RESULTS AND DISCUSSON

in The dynamics of the photogenerated carriers is governed by the electric fields present in the intrinsic region: InGaAs QWs and GaAs barriers. These fields are calculated according to rules described in Ref.[Z], considering InGaAs layers completely clamped to the barriers and substrate, and using the nominal grown values for the In content and layer thickness. The internal fields in (100) and (I 11) samples are both dependent on the applied d.c. bias voltage. Nevertheless, in the (1 I I) samples, two different

Fig. 2. Schematics of the optical pump-electrical probe pulse sequence and circuit set-up. Some time after the internal dipole has been created by the optical pump, a reverse bias pulse applied to the diode changes its internal NAF region to a PAF one. In this way, the remaining dipole charge is extracted as a component added to the exponentially decaying current due to the RC behavior. This current sum is measured on the 50 ohms input impedance of the sampling scope. The component due to the RC behavior alone is measured in the same way just applying the voltage probe without optical pump.

iransient

negative photocurrent

300K

c

0

25

50

,

75

.

300K

-

’

25

465

and out-of-well dipole kinetics

0

I 1

100

.

125

Time (ns)

0

50

25

Time

75

(ns)

Fig. 3. Measured photocurrent transients at different bias voltages for both (100)(a) and (I 1I)(b) samples. For bias voltages lower than -0.7 V the (1 I I) response behaves like (100). positive transient. For voltages higher than -0.7 V it appears a negative photocurrent not previously reported.

agreement with previously reported ones[9]. Then the origin of the completely different transient behavior is due to the change from PAF to NAF state. The negative photocurrent is due to the NAF state, and not to the field existing in each QW that is negative in all the range of voltages used. This demonstrates that the movement of the carriers is governed by the AEF. For samples having the NAF state the photocarriers created in each QW are drifted by the average electric field: electrons (holes) move to the p + (n ’ ) contact. But when they reach the ends of the MQW active region, they find a positive field becoming trapped at the potential minima[lO], thus producing an internal dipole. This dipole modifies the potential distribution trying to flatten the existing NAF. This in turn helps the photocarriers to escape from the potential minima in the conventional direction, giving the observed positive photocurrent. As the forward bias increases the NAF, the charge necessary to flatten it also increases and thus the photocurrent transients show a larger negative response. If the photogenerated charge is not enough lo completely screen the NAF, the transient photocurrent becomes completely negative. This carrier flow and trapping in the intrinsic region creates an external voltage drop of reversed sign, as it is illustrated in Fig. 4. This voltage drop produces a negative photocurrent through the external circuit until such voltage drop disappears. This will be accomplished by a partial screening of ionized donors and acceptors in the space charge edges of the n + and p + regions by their majority carriers, when time has elapsed to allow the flow of such carriers through the external circuit (a few RC time constants). The internal dipole is responsible for the long range screening that lowers the initial average field in the MQW region.

Recombination processes and thermoionic emission will remove the internal dipole. From the same considerations made to discuss Fig. 4, a positive current will appear. This gives the positive final tail in the photocurrent of Fig. 3(b) that becomes slower as the bias voltage is increased. This reflects a decrease of the dipole removal rate as the NAF is more pronounced. For voltages higher than -0.3 V this process become so slow that the positive photocurrent is in the detection limit and all the response looks negative. This suggests a way to study the dipole kinetics by an optical-pump electrical probe technique. Optical pulses (pump) create the charge dipole, and at selected delay times a -2 V step pulse (probe) is superimposed on the d.c. bias voltage, that sweeps (extracts) all the stored charge in the pin active region (Fig. 2). The current waveform over the 50 ohms load has two contributions. One is due to the system RC time constant of the system, and the other is due to the current generated by the extracted charge from the internal dipole. This component due to the stored dipole is, in fact, obtained by subtracting the current transients with and without optical pump pulses after each probe pulse. This current generated by sweeping the internal dipole is shown, as a function of the probing time-delay, in Fig. 5. It is observed that the probe current signal decreases as the delay increases, reflecting the slow vanishing of the internal dipole. For each probe pulse, the time integral of the transient current obtained in this way is proportional to the dipole stored charge (Qdip) remaining at this probing time-delay. The dependence of Q,, with the probing delay is shown in Fig. 6 for a NAF pin device initially biased at +0.25 V. The dipole charge has an initial fast decay, followed by a slow tail that lasts more than

464

I. lzpura el ul.

PIN

N

1

50 n

t m

1i

50R

EQUILIBRIUM

A’+’ =Qdip* Ldip/

E

Fig. 4. Band diagrams of the (111) NAF pin diode in the dark (equilibrium) and just after the light pulse. The charge densities and voltages drops are also shown to explain how the novel negative transient photocurrent appears in these diodes.

100 ps showing a non-exponential time dependence. This can be explained by the increase of the NAF as the screening effect decreases due to the slow dipole destruction. The long lifetimes shown in Fig. 6 are only possible if the electrons and holes are

Punpmba Eons

spatially separated, as it was assumed in our NAF devices. Just after the optical pump, the charge becomes trapped at the extreme QWs (Qd,r). A negative external photocurrent is produced in order to screen a part

delay:

300K

4CQns

0

2t.S

;5-

1ous

$4- .

Bias 0.25 V Pulse

.

4Ous

1mus Blaso.25v

- .. x ?23 fi .

al 2%

Fig. 5. Ditrerential (with and without optical pump) current components extracted by the probe step pulse for different time delays from the optical pump. The probe response decreases as the delay increases.

1

l

’

l

.

.

-2V

.

. -

Pump-Probe Delay (t.6) Fig. 6. Remaining dipole charge as time elapses from the optical pump.

467

Transient negative photocurrent and out-of-well dipole kinetics

of the space charge regions in the p+ and n + layers by an amount Q,,,. The voltage drop due to the condition current comes from charges located at the edges of the space charge region of the pin, and they are separated by a distance equal to the intrinsic region, L,, while the charges of the internal dipole

estimate the piezoelectric field and piezoelectric constant e,, for the 17% In content InGaAs layer. The obtained values agree very well with the recently reported ones at low temperatures. By time-resolved photocurrent and optical-pump electrical-probe measurements at room temperature, we have directly

(Qdlp) are separated by a distance L,,,. The potential changes generated by both charges should be equal

demonstrated that the dipole charges are stored at the ends of the MQW active region, and that the dipole

Pm, x L, = Qap x L, The initial voltage drop measured on the external resistance gives the internal dipole Qdip x Ld,p* as indicated in Fig. 4, where dielectric permittivity constant. Moreover, integral of the negative photocurrent allows

(1) 50 ohms moment E is the the time to deter-

Q,,,, and the time integral of the current extracted by the probe pulse after 50 ns delay allows to estimate Qdlp in a first approximation, since its recombination takes much longer than 50 ns. From such measurements, a dipole charge Qd,, = 5 x 10e9 C cm-’ and a separation Ldlp = 230 nm are estimated from eqn (1) for the internal dipole. This result for Ldip can be compared with the length of 240 nm of the MQW region inside the intrinsic zone of the pin. It confirms the initial idea that in NAF devices photocarriers are stored at the extreme quantum wells of the active region, and it gives direct evidence for the out-of-well screening effects due to the internal dipole. In summary, it has been shown that in InGaAs/GaAs (I 1l)B MQW pin diodes the piezoelectric field produce novel internal electric field profiles. An average field opposite to the built-in one can be generated by a proper device design, favoring charge dipole formation, long range screening of the piezoelectric fields, and the appearance of negative photocurrent transients. The bias conditions for which the negative photocurrent disappears, flat envelope potential in the intrinsic region, allow one to

lifetime exceeds the 100~s range. Acknowledgements-The financial support of CICYT projects TIC93-0026, TIC93-0025 and TIC95-0116 is acknowledged.

REFERENCES

mine

A. S. Pabla, J. L. Sanchez-Rojas, J. Woodhead, R. Grey, J. P. David, R. G. J. Rees, G. Hill, M. A. Pate, P. N. Robson, R. A. Hogg, T. A. Fisher, R. K. Willcox, D. M. Whittaker and M. S. Skolnick. ADDI. Phvs. Lerr. I“

63, 752 (1993). 2

J. L. Sanchez-Rojas. A. Saced&. F. Calle. E. Calleja

and E. Mufioz, Appl. Phys. Len. 65, 2214 (1994). R. Huang, D. R. Harken, A. N. Cartwright, D. S. McCallum, Arthur L. Smirl, J. L. Slnchez-Rojas, A. Sacedbn, F. Gondlez-Sanz, E. Calleja and E. Mufioz, J. Appi. Phyx 76, 7870 (1994). 4. D. R. Harken, X. R. Huang, D. S. McCallum, Arthur L. Smirl, J. L. Sbnchez-Rojas, A. Sacedcin, E. Calleja and E. Muiioz, Appl. Phys. L.e~i. 66, 857 (1994). 5. M. Livingstone, I. Galbraith and B. S. Wherrett, Appl. 3 X.

Phys. iat. 6.

7. 8. 9. 10

65, 2771 (1994).

A. N. Cartwright, D. S. McCallum, Thomas F. Boggess, Arthur L. Smirl, T. S. Moise, L. J. Guido, R. C. Barker and B. S. Wherrett, J. Appl. Phys. 73, 7767 (I 993). E. A. Caridi, T. Y. Chang, K. W. Goossen and L. F. Eastman, Appl. Phys. La. 56, 659 (1990). 0. Madelung (Ed.), Semiconductors, Landolt-B6rnstein New Series, Vol. 17. Springer, New York (1982). J. L. Sanchez-Rojas, A. Saced&, F. Calle, E. Calleja and E. Mutioz, Appl. Phys. Leti. 65, 2042 (1994). J. L. Sanchez-Rojas. A. Saced&. A. Sanz-Her&, E. Calleja, E. Mufioz and E. J. Abril, So/id St. Electron., 40, 591 (1996).