A NEW AIGaAs / GaAs / InGaAs LASING SWITCH GROWN BY MBE

A quantum well optoelectronic switch (QWOES) based on regenerative loop ofpotential barrier lowering resulted from the forward biased pn junction is demonstrated in a A1GaAs/GaAs/InGaAs double heterostructure. Excellent electrical switching characteristics with a high voltage control efficiency (=Vs/VH) of 6.8 have been obtained when the device is operated in the dark. Typical OFF-and ON-state resistances are 120 kft, 25 f, respectively. A lasing threshold current density, front slope efficiency and external differential quantum efficiency measured in as-cleaved device are 210A/cm, 0.4mW/mA and 31.4%, respectively. The peak emission wavelength is centered at about 974nm.

INTRODUCTION Electrical-and optical-induced negative differential resistance (NDR) switching characteristics in III-V semiconductor devices 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 NDR devices, including metal-insulator-semiconductor [1], pn junction [2], and delta-doped superlattice [3] devices, in which they have high speed feature.But, most of them are all concentrated on the investigation of electrical properties.Recently, double heterostructure optoelectronic switches (DOES) have been developed in GaAs/A1GaAs [4], Si/SiGe [5] 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 optoelectronic 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 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 GaAs/InGaAs single quantum well DOES, i.e., QWOES prepared by molecular beam epitaxy (MBE) has been fabricated.The optical thyristor essentially combines the triangular barrier switch and single quantum well (QW) GaAs/InGaAs structure into a DOES.The use of the strained InGaAs layer offer the potential of a lower threshold current when lasing [8], the light electron-hole mass and higher mobility than those of GaAs QW structures.To our knowledge, the QWOES is the first switching device reported to date for GaAs/InGaAs material system with a lasing operation in switching ON-state.

EXPERIMENTAL
The schematic cross section of the QWOES is depicted in Figure (a).The studied structure was grown by MBE on a (100)-oriented n +-GaAs substrate.Si and Be were used as n-and p-type dopants, respectively.This structure contained a 0.5 tm-thick (n + 2 x 10Scm-) GaAs buffer layer, a 0.15 tm-thick (n + =2 x 10Scm -) Alo.4Gao.6Asbarrier layer, a 5nm-thick (p+=l x 109cm -) Alo.4Gao.6Ascharge sheet layer, a 15 nm-thick undopcd Ino.2Gao.aAsQW sandwiched with two 10nm-thick undoped GaAs layers, a 0.081am-thick (n= 1 x 107cm -3) GaAs collector layer, a 0.151xm- thick (p+= 1 x 10Scm -3) Alo.Ga0.6Asbarrier layer and a 0.21xm- thick (p+ 1019cm -3) GaAs cap layer.For fabricating a broad area laser, the p +-side was metallized with Au/Zn and then, the sub- strate was thinned to a thickness of 100 Ixm.Au/Ge was evaporated o.rm n + GaAs on the n-side and the wafer was then alloyed.The area of the laser bar was 150 Im 350 pm after cleaving.Electrical current-voltage (I-V) harateristics were measured by a Tektronix 370A urve tracer at room temperature.

CHARACTERISTICS
The energy band diagram of the QWOES is illustrated in Figure (b).The heterojunction potential barrier and InGaAs QW structure with higher valence band offset are used to improve the localized confinement of the holes.For a DOES, the basic principle of these triangular barrier based switches is that NDR is caused by an increase of hole injection to the potential barrier minimum 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 Figure 2. The measured switching parameters of the QWOES at room temperature are switching voltage Vs= 11 V, switching current Is=0.5 mA, holding voltage VH= 1.6V and holding current I/= 4.5 mA, respectively.I-V characteristics in ON-state are all independent of external voltage due to inherent properties of pn junctions.The resistance in OFF-state is typically FIGURE 2 Room temperature I-V characteristic of the QWOES.
greater than 120 KF/and the ON-state resistance is lower than 25 f.The observed switching and holding voltage lead to a high voltage control efficiency, r/v (= Vs/VH), of 6.8.This voltage control efficiency is among the largest reported so far in a field represented by DOES [4][5][6].
The optical characteristics of the as-cleaved lasing QWOES were measured under pulsed condition at room temperature.In the case of pulsed operation, the pulse width was 50 Ixs with a repetition rate of 1 KHZ. Figure 3 shows the pulsed power output versus current and spectrum Onset) measured at a bias current equal to the threshold current.The threshold current I was measured to be 110 mA, with a front slope efficiency of 0.4 (mW/mA), and yielded 14 mW, at a current of 134 mA.The external differential quantum efficiency is as high as 31.4%.Although the Ith is large, it is the first experimental realization of a lasing device in GaAs/InGaAs-based DOES.Hence, it is pointed out that this device has not been optimized, better laser cavity geometries are expected to allow the decrease of Ith.In addition, the spectrum of the optical output is also shown in the inset of Figure 3, at a current I Its.As can be seen, the QWOES lases at an emission wavelength about 974 nm with a spectrum full width at half maximum (FWHM) of 1 rim.

CONCLUSIONS
We have successfully exploited for the first rime an InGaAs QW infrared laser diode operating at a wavelength of 974 nm on switching using AlGaAs/GaAs/InGaAs double heterostructure.The lasing threshold current is l0mA, corresponding to a current density of 210 A/cm 2, and the external differential quantum efficiency is 31.4% at FIGURE (a) Schematic cross section of the QWOES (b) Energy band diagram of the QWOES at equilibrium.

FIGURE 3
FIGURE 3  Pulsed power output versus current and emission spectrum (in the inset) measured at a bias current I Ith.