LASING THYRISTOR (QWLT) PREPARED BY MOLECULAR BEAM EPITAXY

Transcription

1 Active and Passive Elec. Comp., 1998, Vol. I, pp Reprints available directly from the publisher Photocopying permitted by license only (C) 1998 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in India N-CHANNEL InGaAs QUANTUM WELL LASING THYRISTOR (QWLT) PREPARED BY MOLECULAR BEAM EPITAXY H.C. CHEN Department ofelectrical Engineering, Far-East Junior College of Technology and Commerce, Tainan, Taiwan 744, Republic of China (Received 1 7 June, 1996) N-channel lngaas quantum well lasing thyristors (QWLT) based on regenerative loop of potential barrier lowering resulted from the forward biased pn junction is demonstrated in a AIGaAs/GaAs double heterostructure. Excellent electrical switching characteristics with a high voltage control efficiency rlv (--Vs/VH) of 6.7 have been obtained when the device is operated in the dark. Typical OFF- and ON-state resistances are 150K.Qand 10f respectively. A lasing threshold current density, front slope efficiency, and external differential quantum efficiency measured in as-cleaved device are 183A/cm2, 0.4mW/mA, and 31%, respectively. The peak emission wavelength is centered at about 980nm. 1. INTRODUCTION III-V semiconductor negative differential resistance (NDR) devices with electrical- and optical- induced switching characteristics have found applications in a variety of areas such as microwave generation and high-frequency oscillation. Of particular interest is the development of S-shaped NDRdevices,including metal-insulator-semiconductor 1, pn junction2, and delta-doped superlattice 3 devices, in which they have high-speed features. However, most studies have concentrated on the in- vestigation of electrical properties. Recently, double heterostructure optoelectronic switches (DOES) have been developed in GaAs/A1GaAs4, Si/SiGe5, and InP/InGaAsP 6 material structures, which are all mainly based on an inversion charge sheet at the internal heterojunction interface. The combination of both electrical and optical switching properties has become a promising feature for optoelec- 73

2 74 H.C. CHEN tronic integration. 7 For a DOES constructed by a direct band-gap material system, the impedance is high and there is no emission of light in the OFF-state. However, the impedance is low and strong emission of light occurs in ON-state. The switching phenomenon between two states could be induced by either electrical or optical triggering. In this work, a new n-channel InGaAs single quantum well DOES, i.e., QWLT prepared by molecular beam epitaxy (MBE) has been fabricated. The optical thyristor essentially combines the triangular barrier switch and single quantum well (QW) GaAsflnGaAs structure into a DOES. The use of the strained InGaAs layer offers the potential of a lower threshold current when lasing8, the lighter electron-hole mass, and higher mobility than those of GaAs QW structures. To our knowledge, the QWLT is the first switching device reported for a GaAs/InGaAs material system with a lasing operation in the switching ON-state. 2. EXPERIMENTAL The schematic cross section of the QWLT is depicted in Fig. 1. The structure was grown by MBE on a (100)-oriented n/-gaas substrate. Si and Be were used as n- and p-type dopants, re- spectively. This structure contained a 0.51xm-thick (n+=2xl018cm-3) GaAs buffer layer, a 0.15txm- thick delta-layer 0.2m p+-. GaAs 1E19 cm prn p+_ Al,,Gao,As 1518 cm-3 Snm (()-AI Ga As 1519 cm " ion i GaAs 15nm i_lno,ga As 10nm GaAs 0:15/5 m p.-gaas 1517 cm ,rn N *- AIo.,Ga=8. As 2E18 cm "a 0.5/Jm n+- GaAs 2E18 cm.3 n+-- GaAs Substrate FIGURE Schematic cross section of the n-channel QWLT

3 QUANTUM WELL LASING THYRISTOR 75 (n / 2 x 1018cm-3) Alo.4Gao.6As barrier layer, a 0.15gm-thick (p x 1017cm-3) p-gaas layer, a 15nm-thick undoped Ino.2Gao.8As QW sandwiched with two 10nm-thick undoped GaAs layers, a 5nm-thick (n / 1 x 1019cm-3) Alo.4Gao.6As charge sheet layer, a 0.15gm-thick (p/ x 1018cm-3) Alo.4Gao.6As barrier layer, and a 0.2gm-thick (p/ x 1019cm3) GaAs cap layer. For fabricating a broad area laser, the p/-side was metallized with Au/Zn and then the substrate was thinned to a thickness of-- 100gm. Au/Ge was evaporated on the n-side and the wafer was then al- loyed. The area of the laser bar was 200gmx300gm after cleaving. Electrical current-voltage (I-V) characteristics were measured by a Tektronix 370A curve tracer at room temperature. 3. RESULTS AND DISCUSSION The A1GaAs/GaAs heterojunction potential barrier and InGaAs QW structure with higher valence band offset are used to improve the localized confinement of the holes. For a-n-channel DOES7, the basic principle of these triangular barrier-based thyristors is that NDR is caused by an increase of electron injection to the potential barrier maximum with an increase in the forward biased pn junction; then the triangular barrier rapidly collapses, leading to a fast switch from a high-impedance OFF-state to a low-impedance ON-state. The experimental I-V characteristic at room temperature is shown in Fig. 2. The measured switching parameters of the QWLT at room temperature are switching voltage Vs=12.8V, switching current Is=80gA, holding voltage VH=I.8V, respectively. With increasing applied voltage I H is 17mA, while with decreasing applied voltage, I H drops to 4.4mA. This hysteresis is a result of electrical oscillations in the device, caused by the interaction of the NDR with parasitic circuit resistance. 9 I-V characteristics in the ON- state are all independent of external voltage due to inherent properties of pn junctions. The resistance in the OFF-state is typically greater than 150Kf and the ON-state resistance is lower than 10D, The observed switching and holding voltage lead to a high voltage control efficiency, fly (= Vs/VH), of 6.7. This voltage control efficiency is among the largest reported so far in a field represented by DOES. 4"6 The optical characteristics of the as-cleaved lasing QWLT were measured under pulsed con- dition at room temperature. In the case of pulsed opera-

4 76 H.C. CHEN tion, the pulse width was 501.ts with a repetition rate of 1KHz. Fig. 3 shows the pulsed power output versus current and spectrum (inset) measured at a bias current equal to the threshold current. The threshold current Ith was measured to be 110mA, with a front slope efficiency of 0.4(mW/mA), and yielded 14mW, at a current of 134mA. The extemal differential quantum efficiency is as high as 31%. Although the Ith is large, it is the first experimental realization of a lasing device in n-channel A1GaAs/GaAs/InGaAsbased DOES. Hence, it is pointed out that while this device has not been optimized, better laser cavity ge- ometries are expected to allow the decrease of Ith. In addition, the spectrum of the optical output is also shown in Fig.4, at a current I=l.lIth. As can be seen, the QWLT lases at an emission wavelength about 980nm with a spectrum full width at half maximum (FWHM) of 2nm. VERT/D V 2mA CURSOR HORIZ/DIV 2V CURSOR PER STEP 5On^ OFFSET O. On^ B 9m/DIV 40K X OF COLLECTOR PEAK VOLTS "gP. 370A AUX SUPPLY 0,00 V FIGURE 2 Room temperature I-V characteristic of the QWLT

5 QUANTUM WELL LASING THYRISTOR Pulse Width: 50 psec Rate: looohz Re=petition O0 150 Current (ma) FIGURE 3 Pulsed power output versus current of the QWLT Pulse Width: 50 Repetition Rate: T=3OOK,,t,1, I... <Tick Spacing is lnn> 991inn FIGURE 4 Emission intensity versus wavelength of the QWLT measured at a bias current I=1. llth

6 78 H.C. CHEN 4. CONCLUSIONS We have successfully exploited for the first time an n-channel InGaAs QW infrared laser diode operating at a wavelength of 980nm on switching using A1GaAs/GaAs/InGaAs double heterostruc- ture. The lasing threshold current is 110mA, corresponding to a current density of 210A/cm2, and the external differential quantum efficiency is 31% at room temperature. I-V switching charac- teristics are excellent and reproducible with a high voltage control efficiency of 6.7. The inherent properties of electrical bi-stability and laser emission suggest the possibility of the QWLT for optoelectronic applications. References [1] T. Yamamoto and M. Morimoto:"l hin-mis-structure Si negative-resistance diode, AppL Phys. Lett., 20, 269 (1972) [2] C.E.C. Wood, L.F. Eastman, K. Board, K. Singer and R.J. Malik: "Regenerative switching devices using MBE-grown gallium arsenide, Electron. Lett., 18, 676 (1982) [3] E.E Schubert, J.E. Cunningham, and W.T. Tsang: Perpendicular electronic transport in doping superlattices, Appl. Phys. Lett., 51, 817 (1987) [4] G.W. Taylor, J.G. Simmons, A.Y. Cho, and R.S. Mand: A new double heterostructure opto-electronic switching device using molecular beam epitaxy, J. Appl. Phys., 59, 596 (1986) [5] S.J. Kovacic, J.G. Simmons, K. Song, J.P. Noel, and D.C. Houghton: Si/SiGe digital opto-electronic switch, IEEE Electron Device Lett., EDL-12, 439 (1991) [6] S.J. Kovacic, B.J. Robinson, J.G. Simmons, and D.A. Thompson: InP/InGaAsP double-heterostructure optoelectronic switch, IEEE Electron. Dev. Lett., EDL-14, 54 (1993) [7] G.W. Taylor, R.S. Mand, A.Y. Cho and J.G. Simmons:"Experimental realization of an n- channel double heterostructure optoelectronic switch", Appl. Phys. Lett., 48, 1368 (1986) [8] S.W. Corzine, R.H. Yan, and L.A. Coldren: Theoretical gain in strained InGaAs/GaAs quan- tum wells including valence-band mixing effects, Appl. Phys. Lett., 57, 2835 (1990) [9] S.J. Kovacic, J.J. Ojha, J.G. Simmons, P.E. Jessop, R.S. Mand and A.J.S. Thorpe, "Optical and electrical oscillations in double heterojunction negative differential resistance devices", IEEE Trans. Electron. Dev., ED-40, 1154 (1993).

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