GaAlAs laser diode

Solid-State Electronics Vol. 30, No. I, pp. 53-51, Printed in Great Britain. All rights reserved

OPTICAL

1987 Copyright

BISTABILITY IN A pnpn LASER DIODE

0038-I 101/87 Q 1987 Pergamon

$3.00 + 0.00 Journals Ltd

GaAs/GaAlAs

WANG SHOU-WV, WV RONG-HAN, ZHANG QUAN-SHENG and Hu DAN-XIA The Institute

of Semiconductors, (Received

21 January

Chinese

Academy

of Sciences,

1986; in revisedform

Beijing,

China

4 July 1986)

Abstract-In this paper, we report for the first time some experimental results on the optical bistability and switching characteristics of a pnpn negative resistance laser diode. The related physical explanation of the optical bistability of the device based on the double transistor model and charge storage theory is given.

1. INTRODUCI’ION

The output can be either optical, electrical or both. The light output is from the end face of d2 region.

With rapid development of optoelectronics and its applications, much attention has been paid to the research of optical bistability[l-141. The recent interest in this phenomenon is largely due to the potential applications of the device for the control of optical signals being performed in the optical domain, functions analogous to those of various electronic devices. In this paper, we report some experimental results on optical bistability and switching characteristics in a pnpn laser diode developed by the authors[l5,16]. The diode used to demonstrate the optical bistability can be considered as an active optoelectronic hybrid bistable device in which the optical bistability comes from the nonlinearity of the electric amplification parameter with a positive electric feedback under illumination. A physical description is given of the device operating in a bistable mode.

2.

THE ANALYSIS OF THE DEVICE UNDER BISTABLE OPERATION

The structure of a broad contact GaAs/GaAlAs pnpn laser diode used in the experiment, with 6 layers grown by the LPE technique, is schematically shown in Fig. 1. In accordance with the LPE growth sequence the 6-layer structure can be denoted simply as N-Ga,_,Al,As/p-GaAs (active)/P-Ga,_,Al,As/ p-GaAs/n-GaAs/P-Ga,_,Al,As, i.e. N-p-P-p-n-P (the capital letter is for GaAlAs). The unique feature of this device is that the lasing threshold current density is low, and the good lasing properties are with well turned-on electrical compatible characteristics[ 151. The lasing properties are similar to that of a normal DH pn laser. For demonstrating the optical bistability, the input light with wavelength 1 = 8200 A longer than the band gap of Ga,_,Al,As enters the top side of the Ga,_,AlzAs layer and is absorbed mainly in n-GaAs (d,) and p-GaAs (&).

P Go ,_,AIzAs z=o.1

”

- 5 x 10’BCm-3 d6=3pm

Illlllll

- IO” d5=05

GoAs

x2 0.3

d,=4

” GOAS (substrate)

Fig. 1. Six-layer 53

device structure.

WANG Snou-wu

-

Fig. 2. Simplified

2nd

double

transMzr

transistor

-

model

of the

device

structure.

In darkness, the device can be considered as a mutually related double transistor with the common base short circuit current amplification factors for the first and second transistor illustrated in Fig. 2 as follows

1 -I

1 +$eAEy/KT

ctO,=

[

,

2

(1)

where d2, d,, d5 and L,, L, are the thickness and diffusion length of minority carriers in the corresponding layers and AEyr the barrier height for electrons in the lasing active layer (d,). With our design, a02 is nearly equal to unity over a very wide current range, and clO,is very small, almost zero. In fact, we make tlO,+ a02 a little bit less than 1 so that the device under certain bias conditions remains in the off state in darkness. Since equations (1) and (2) are only approximate expressions for ~6, and c+,~,and they increase slightly with device current in practice, the device will be switched from the off state to the on state as soon as the following condition is fulfilled c&+cc,,=

1

et

al

state is less under illumination than in darkness. Owing to the different initial conditions in the turnon and turn-off routes, under the given DC bias condition, the input light intensity required for turning the device on and off are different, resulting in the optical bistability. For further description about the optical bistable operation of this diode, a family of dynamic V-Z characteristics with variable light input are shown schematically in Fig. 3. A detailed analysis of the V-I characteristics for the device in darkness has been given previously[l6]. It should be noticed however, that the details of the dynamic V-Z curve depend more or less on the external circuit parameters (i.e. series resistance and shunt capacitance in the external circuit) used in the measurement, while the turn-on voltage Vs and the holding current Z, in darkness are characteristics of the device itself. The slope of line ef in Fig. 3 corresponds to the equivalent series resistance of the device in the on state. Supposing a bias voltage less than Vs is applied to the device with a load resistance connected in series with it, a load line cd can be drawn in the V-I diagram as shown in Fig. 3. When the intensity of the incident light P increases gradually from P,, to P, and then to P,. The device remains in the off state, but the operating point of the device moves along the load line from point Y to s and then to t. If the P increases further from P2 to P,, the device will suddenly be switched on. As soon as the device is switched on, the operating point of the device moves rapidly to point u and will stay there as P further increases. Now if P is decreasing, the device will stay in the on state until it approaches P, . Since the ordinate of point u is smaller than I,, , the device will be switched off before P reaches P,.

(3)

The device can be switched on electrically or optically. It can remain in darkness and the switching process may be triggered by an additional forward electrical pulse. On the other hand, when the device is illuminated by a beam of light, a rather large photocurrent will pass through the device even if it is in the off state. The photocurrent is always in the same direction as the bias current, thus it makes the switching condition [i.e. equation (3)] for the device (using double transistor model) to be satisfied at a much lower bias current level. When the switching process from the on state to off state is considered, the reverse situation applies. The minimum bias current for holding the device in the on

C

e

d

%

v

Fig. 3. Dynamic V/I characteristics with different incident light (schematically). Holding current I,,, I,, I2 and I, corresponding to the incident light power P. (= 0), P, , Pz, P,, separately, and P, i P2 i P,.

Optical bistability in a pnpn GaAs/GaAlAs laser diode It is clear that, if P, < P I,,, the device can be switched on by a light pulse, but will never be switched back to the off state again unless the bias condition is changed. If Z < Z,, the device works in a light amplifier mode. In the following, the double transistor mode will be used for estimating the influence of illumination on the holding current, which is important in measuring optical bistability performance. Similar to the mathematical treatment in Ref.[l6], the charge storage equations are given as follows:

at steady state condition,

(8) (9) where, G=~-l=~“p,-l, “P

and G is the excess loop gain with /3, = s,,/t, and BP= zp/tp being the common emitter amplification factor respectively for the 1st and 2nd transistor. The total current going through the diode at on state is given by Z=g+g+P(a+b) tn t/I

s+(;+$z-(&-&)Qn (6)

=

PM1 - ao1)+ 41 - a,,)1 1 - a01- a02

The above quadratic equation for Z has two roots namely: =h+ap

z

I

+JVh-ap)*-4aplA 2

z =Zh+ap-d(I,-ap)*-4aPIA 2 2

( r”

n>

(7)

(12) (13)

Z, corresponds to the holding current in the on state, while Z2is the saturation photocurrent in the off state. From equations (12) and (13) it can be easily seen that:

z, > z, > ZJ2;

E+p,

(11)

From equation (1 l), it can be seen that in darkness (P = 0) Z is always equal to zero except when tlo, + q, = 1. This means that the device in darkness will remain in the off state (the saturation current Z, is assumed to be negligible as mentioned above) unless the condition tool+ cloz= 1 is fulfilled. On the other hand, when the device is under illumination, P is no longer equal to zero and Zcan be very large even if (1 -a,,, - a,,r) is very small. This means that the device can be switched on when (1 - tlo, + or) is sufficiently small but not equal to zero. We assume that the variation of (1 - aol + cto2)is proportional to (I* - Z) in the small range of current near the steady on state, so that 1 - tlo, - ao2= A(Z,, - I) with A being a constant, and taking a = 6, equation (11) becomes:

Z, = Z,,; Z2= 0 for P = 0 (in darkness) =p

(10)

By using Q,, Q, of equations (8) and (9) in equation (IO), the following expression can be established: z

where the symbols are; Q,, electron charge in the base of the first transistor, Qr, hole charge in the base of second transistor, t,, z,, transit time and mean lifetime of electrons as minority carriers in the base of transistor 1, tp, zp, transit time and mean lifetime of holes as minority carriers in the base of transistor 2, P, incident photon flux, a, 6, constants related to the photo-generated carriers and assuming the photocarriers generated near the reverse bias region give the main contribution to the photocurrent. It should be noticed that the minority carrier charges in the base regions of both transistors under thermal equilibrium condition have been ignored in equations (4) and (5), because they are very small in comparison with either the injected or the photogenerated carrier charges in the respective regions. This approximation is equivalent to ignoring the saturation current of the reverse biased pn junction in darkness, in comparison with the holding current of the device. Combining equations (4) and (S), we obtain:

55

Z, > I2 > 0 for P # 0 (under illumination)

WANG Snowwu

56

Fig. 4. Steady V/I characteristics X 2 V/cm Y 1ma/cm, light input (from right to left) 0, 100, 170, 400, 550, 85OpW. Besides, as the incident light intensity P increases, I, will decrease while I, increases, and eventually I, and I2 approach each other under high-intensity illumination.

3.

EXPERIMENTAL ASSESSMENT DISCUSSION

AND

The experimental devices are of broad contact type with the light (2 = 0.82 pm) incident upon the top face and absorbed in the region of the two bases (d4, d,). The light output from the cavity’s lateral face (dz) is detected. The family of S-type forward P’/Z characteristics at steady state is shown in Fig. 4 for different incident light. Typically device parameters under electrical operation in darkness are as follows: turn on turn on holding holding

voltage current voltage current

(V,) (1,) (V,) (I,,)

et al.

capacitance (C) or input light power. Some investigations on the frequency characteristics of the device have been made previously by authors[ 161. d.c. and pulse optical bistability observation. Figure 6 shows the experimetnal optical bistable traces under d.c. light input with bias current satisfying IL < I < I,,. In that case the device shows good bistable characteristics with controllable span and clearly distinguished bistable states. The output can be either electrical or optical, or both; this is a very desirable feature from the point of view of applications. Traces l-5 in Fig. 6 show clearly the bistable span with different applied bias in the range of IL < I < I,. Trace 6 illustrates the case when the applied current I > &. The output from the device is spontaneous emission because the holding current in the dark is smaller than the threshold current in our case. However, if the structure parameters are modified to further increase the holding current, possibly lasing emission in bistable operation will be attainable. In Fig. 7, a pulse optical bistable oscillographic trace displays in fact an optical data processing function of the device. The applied electrical and optical conditon is similar to that in d.c. The input ladder light pulse can be seen as an overlap of two pulses with different pulse height, neither of them can

10-20 V, 50-100 @A, 1.4 V, 10-200 mA.

From this figure, it can be seen that as the incident light increases, both the turn on voltage and holding current decrease. Besides, the holding voltage stays very low, implying the junction power dissipation is small during the on state, and the designed dark holding current can be variable over a large current range, making the bistable output easily modulated. For demonstrating an optically triggering switch laser action, we put the diode in to a so-called self-oscillation circuit with the forward bias across the diode lower than P’,. With the light on, if the current offered by the circuit itself at the on state is larger than the threshold current, lasing action within the relaxation oscillation can be obtained. This is the case in our measurements because of the low lasing threshold current density (J,,, N 2500 A/cm2) and the large on state current. Figure 5(a) and (b) illustrate the self-oscillation circuit and the lasing far field pattern during self-oscillation under illumination. The self-oscillation frequency can be regulated by varying the applied voltage (V), resistance (R),

0

(a)

(b) Fig. 5. (a) self-oscillation circuit (V < V_ V/R
Optical bistability in a Pnpn GaAs/GaAlAs

Light input (mW)

laser diode

sessed. The output response time depends on the incident light intensity and bias current; with increasing incident light and current, the rise time decreases, while the fall-time increases. With a high intensity rectangular incident light pulse, we obtain an electrical output rise-time in the range of nanosecond (the minimum is 4 ns depending on the measuring condition), and a fall-time in the range of 10 ns. Analysis shows that the time characteristics can be improved, by decreasing the area of sample, junction capacitance, or applying a pre-bias to the device. The experimental results mentioned above and reported before in Refs[lS] and[l6] indicate that the GaAs/GaAlAs pnpn laser diode is a device with many functions including light emission, optical bistability, switching and light detection, which vest the device with a potentiality for applications such as high speed light source, optical pulse shaper, optical switching, signal processing and optically controlled memories.

Fig. 6. Optical bistability traces under d.c. illumination operation, X light input, Y light output (arb. units), l-5 bistable mode (r, 15, 21, 24, 26, 27 ma), 6 switch mode [I > I,(30 ma)].

turn the diode on. Only with the combination of both pulses does the device turn from the off state to the on state. It remains in the on state even if either pulse is removed. Among other things, the device acts as an optical shaper, if a single optical pulse illuminating

the device is strong enough. The time characteristics for such a demonstrated sample (300 pm long, 100pm wide) has been as-

57

REFERENCES

1. H. M. Gibbs, S. S. Tarng, J. L. Jew& D. A. Weinberger and K. Tai, Appl. Phys. Left. 41, 221 (1982). 2. H. M. Gibbs, S. L. McCall, T. N. C. Venkatesan, A. C. Gossard, A. Passner and W. Wiegmann, Appl. Phys. Lerr. 35, 451 (1979).

3. D. A. B. Miller, D. S. Chemla and T. C. Damen, Appl. Phys. Left. 45, 13 (1984).

4. H. M. Gibbs, S. L. McCall and T. N. C. Venkatesan, Phys. Rev. Letf. 36, 1135 (1976). 5. Hitoshi Kawaguchi, ZEE PROC. 129, 141 (1982). 6. S. D. Smith and A. C. Walker, ICO 13 B5-10, 447 (1984). 7. C. Harder, K. Y. Lau and A. Yariv, IEEE J. Quantum Electron. QE-18, 1351 (1982). 8. K. Y. Lau and A. Yariv, Appl. Phys. Leti. 45, 719 (1984). 9. Y. C. Chen and J. M. Liu, Appl. Phys. LetI. 46, 16 (1985).

10. J. M. Liu and Y. C. Chen, IEEE J. Quantum Electron, QE-21, 298 (1985). 11. I. H. White and J. E. Carroll, IEE PROC. 131, 309 (1984). 12. K. Okumura, Y. Ogawa, H. Ito and H. Inaba, IEEE J. Quantum

Elecwon.

QE21,

377 (1985).

13. B. S. Ryvkin, Sov. Phys. Semicond. 19, 1 (1985). 14. Wang Qi-ming, Zhu Long-de and Cao Qi-ping, Acra physica sinica 34, 1102 (1985). (in Chinese). 15. Wang Shou-wu, Wu-Rong-han, Zhu Qi-gao, Zhang Quan-sheng, Li Zhou-yin and Tian Hui-liong, IEE Fig. 7. Optical bistability oscilloscope trace under pulse illumination operation, upper light output (arb. units), lower light input X 10 pS/div, Y 0.5 mW/div.

PROC.

129, 306 (1982).

16. Wang Shou-wu, Zhang Quan-sheng, Li Zhao-yin and Wu Rong-han, IEE PROC. 132, 69 (1985).