SEMICONDUCTOR LIGHT EMITTING DEVICE

;03D29D08D3 # 3/ 21 SEMICONDUCTOR LIGHT EMITTING DEVTOK FIELD OF THE INVENTION The present invention relates to a semiconductor light emitting device and, more particularly, to a semiconductor light emitting device, such as semiconduclor laser, suitable for lise in an optical communication system as an optical amplifier having a low waveguide loss.

BACKGllOUNl) Semiconductor lasers made of InGaAs based compound semiconductors (referred to as InGaAs is based semiconductor lasers) and having a wavelength of 980 nm range are widely used as a variety of light sources, such as a pumping light source in an optical fiber amplifier. Fig. 3 shows a conventional TnGaAs based semiconductor laser having a 980-nm-range wavelength in a cross- section.

The conventional semicouduclor laser, as shown in Fig. 3, generally designated by numeral 10, includes a 100- //. m-thick n-type GaAs (n-GaAs) substrate 1, and epitaxial layers consecutively Received Oct-13-OQ 01:40am Froni-03B2860BB3 To-Smart I Biggar Page 003 00-10-13:14:31 ;1**fi?F*M FAX03-D29D-0853 ;03D29D08D3 It.

grovrn thereon. The epitaxial layers includes, consecutively from tlie substrate 1, a 1.7-Mm-thick n-AlOaAs cladding layer 2, a 30-nm-thick undoped AlGaAs SCH layer 3, InGaAs/GaAs quantum well 4, a 30-nm-thick undoped AlGaAs SCH layer 5, a 1.7- /£m-thick p-AlGaAs cladding layer 6, and a 0.3- U - thick p-GaAs cap layer 7.

The cap layer 7 and the top portion of the cladding layer 6 are configured as a 4- At m-wide mesa stripe. A passivation layer 8 made of SiN is formed on the area other than the top surface of the cap layer 7. A p-side electrode 9 made of Ti/Pt/Au metallic layers is formed on the lop of the passivation layer and an n-side electrode 10 made of AuGe/Ni/Au metallic layers are formed on the bottom surface of the substrate 1.

The conventional TnGaAs based semiconductor laser device 20 has a bandgap energy Egl of 1.41 eV at the n-GaAs substrate 1, and a bandgap energy Eg2 of 1.27 eV at the active layers 4, wherein Egl>Eg2. Thus, the light having a bandgap energy Eg2 can pass through the n-GaAs substrate.

Fig. 4 shows the semiconductor laser device of Fig. 3 at a fabrication step thereof. For fabrication of the semiconductor laser device 20, Received Oct-13-00 01:40am Fron-036Z060863 To-Smart t Biggar Page 004 00-10-13;14:31 ;ft9ftNffiffi FAX03-5Z9D-08D3 ;Û3SZ3D08D3 li> first, a 1.7-Mm-thick n-AlGaAs cladding layer 2, a 30-nm-tliick undoped AlGaAs SCH layer 3, TnGAa/GaAfi quantum well active layers 4, a 30- nm-thick undoped AlGaAs SCH layer 5, a 1.7- jU m- thick p-AlGaAs cladding layer 6, and a 0.3-# -thick p-GaAs cap layer 7 are consecutively grown on a n- GaAs substrate 1 by a metal-organic chemical vapor deposition (MOCVD) technique.

Subsequently, the cap layer 7 and the top portion of the cladding layer 6 are etched to form a 4- m m-wide mesa stripe, followed by deposition of a passivation layer H made of RiN on the cap layer 7.

Thereafter, a portion of the passivation layer 8 on the top of the mesa stripe is etched to form an opening for injection of current.

Further, the bottom of the n-GaAs substrate 1 is polished to obtain a total thickness of 100 U m for the substrate 1, followed by evaporation of metals on the passivation layer 8 and the bottom of the substrate 1, to form the p-side Ti/Pt/Au electrode 9 and the n-side AuGc/Ni/Au electrode 10.

Thus, the InGaAs based semiconductor laser 20 of Fig. 3 can bo achieved.

The conventional InGaAs based semiconductor laser has a disadvantage in that the semiconductor Received Oot-13-00 01 MQam Froni-03B20508B3 To-Smart I Biggar Page 006 00-10-13:14:31 ittiSSiM FAX03-D29Q-0803 ;03Q23D08D3 laser has a large waveguide loss, which causes a larger threshold current for lasing and thus a lower lasing efficiency. The large waveguide loss was confirmed by fabricating the semiconductor laser of is Fig. H and another semiconductor laser having a structure similar to the semiconductor laser of Fig.

3 except for the active layers, which were made of GaAs/AlGaAs based compound semiconductors and lasing at a wavelength of 860-nm range in the io another semiconductor laser. The another semiconductor laser had a waveguide loss lower than the waveguide loss of the conventional InOaAs based semiconductor laser by several unils in terms of cm"1.

is SUMMAKY OF THE INVENTION It is therefore an object of the present invention to provide an InGaAs based se mi conductor laser having a lower threshold so current for lasing and thus a higher lasing efficiency.

The present invention provides an InGaAs based semiconductor laser including a semiconductor substrate having a first conductivity- type, and a first cladding layer having the first Received Oct-13-00 OMOam From-0362QB0863 To-Smart t Biggar Page 006 00-10-13:14:31 ;tt9fôFiflfRI FAX03-D23 D-08 D3 ;03529G0803 7/ 21 m conductivity-type, an active layer structure for lasing at a first wavelength, a second cladding layer having a second conductivity-type opposite to the first conductivity-type, and a cap layer having the second conductivity-type, which are consecutively formed on the semiconductor sxibstrate, the first wavelength being larger than a bandgap wavelength of the semiconductor substrate, an effective refractive index of a light emitting waveguide mode being lower than a refractive index of the semiconductor substrate, characterized in that a waveguide loss generated by leakage of light power from the first cladding layer to the semiconductor substrate and from the second cladding layer to the cap layer in a leaky mode is lower than a modal gain of the semiconductor light emitting device.

In accordance with the InGaAs based semiconductor light emitting device of the present invention, the lower waveguide loss generated by the leaky mode of light power compared to the modal gain of the semiconductor light emitting device provides a lower threshold current for light emission and thus a higher emission efficiency of the semiconductor light emitting device.

Rtesivad Oct-13-00 01:4Bam From-03629G0B63 To-Smart 4 Biggar Page 007 00-10-13;14:31 ;fàfâ8f1M FAX03-D29 D-0853 ;03D23D08D3 The present invention can be applied to both a facet omission semiconductor laser and a surface emission semiconductor laser so long as the raltionship between the effective refractive index * (Vett) of the la sing waveguide mode and the refractive index (nflUh) of the substrate is neff<nuub.

The present invention is also applicable to any of GaAs, InGaAy, InGaNAs and InP based light emitting devices irrespective of the compositions of the semiconductor substrates and the active layers.

In a preferred embodiment, the waveguide loss generated by the leaky mode of the light power at the lasing wavelength is below 3 cm"'. This is achieved by the configuration wherein the overall confinement coefficient of waveguide layers including the first-conductivity-type cladding layer, the active layer and the second-condur.tivity-type cladding layer is 90% or more. If the lasing wavelength is 9H0-nm range, a pair of SCH layers acting as a part of active layers may be provided for sandwiching therebetween the active layer structure. In addition, the thickness of the first and the second cladding layers may be 2 Mm or more.

The active layer structure may include a quantum well structure and a pair of SCH (Separate Received Qst-13-OQ 01:49am Fron-0362Q60853 To-Smart 4 Biggar Page 009 00-10-13:14:31 ifâfôfrfcM FAX03-529D-0803 ; 03 029 50803 # 9/ 21 Confinement Hetero-structure) layers sandwiching therebetween the quantum well structure. The quantum well structure may include indium or nitride.

n If each of the cladding layers is made of AlxGaAs, and each of the SCH layers is made of AlyGayAs, the following relationships should hold:

0<xl7 0y<l and x>y.

Therefore, if we choose suitable values for x and y io satisfying the above relationships, we can control two confinement coefficients independently of each other, which are used for the quantum well layer and for the active layers including SCH layer and two cladding layers.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of an InGaAs based semiconductor laser according tu au embodiment of the present invention.

Fig. 2 is a graph for showing relationship between injected current and an optical output of the semieonduelor laser of Fig. 1.

Fig. 3 is a cross-sectional view of a conventional InGaAs based semiconductor laser.

as Fig. 4 is a cross-sectional view of the InGaAs Rsceived Oct-13-00 01:40am F r oiiH)3E2B50863 To-Smart I Bigaar Page DOS 00-10-13:14:31 ;ttSIS*M FAX03-D23D-08Q3 ;03D23D08D3 # 10/ 21 based semiconductor laser of Fig. S at a fabrication step thereof.

IVig. 5 is a graph for showing relationship between injected current and an optical output of the semiconductor laser of Fig. 4.

PREFERRED EMBODIMENTS Before describing preferred embodiment of the present invention, the disadvantage of the iu conventional InGaAs based semiconductor laser will be discussed for a better understanding of the present invention.

It is known that the causes of the waveguide loss studied heretofore include the optical is absorption by the substrate. This is considered to result from the fact that if the substrate is opaque against the lasing wavelength of the semiconductor light emitting device, the light emitting mode is subjected to the optical absorption by the substrate to have a waveguide loss when the optical electric- field in the light emitting mode reaches the substrate.

Since the GaAs substrate is opaque against the lasing wavelength of the GaAs/AlGaAs based semiconductor laser, according to the conventional Reoeivsd Oot-13-00 OMOam Froffl-0362860B63 To-Smart I Biggar Page 010 00-10-13:14:31 iftSftfFÉM FAX03-D29 5-0803 ;03029D0803 # 11/ 21 theory for the 0|)tical absorption, the light emitting mode is subjected to the light absorption by the substrate to have a waveguide loss when the optical electric field in the light emitting mode reaches the n GaAs substrate. On the other hand, since the GaAs substrate is transparent for the lasing wavelength of the InGaAs based semiconductor laser, it is considered that the light emitting mode is not affected by the light absorption from the jlo substrate and the waveguide loss is not increased even if the optical electric field of the light emitting mode reaches the GaAs substrate.

However, according to our experiments for the waveguide loss, it was confirmed, contrary to the is study based on the conventional theory for the optical absorption by the substrate, that the waveguide loss of the InGaAs based semiconductor laser was higher than the waveguide loss of the GaAs/AlGaAs based semiconductor laser, which implies that there is unknown cause fox- the waveguide loss other than the optical absorption by the substrate.

The inventors thus analyzed the waveguide loss to finally find a cause of the waveguide loss, which originates from the fact that the waveguide Received Oct-13-O0 OlMOam From-03620E0863 To-Smart t Biggar Page Oil 00-10-13:14:31 ifififtSÉM FAX03-S23 5-08D3 ;03D23D0653 loss caused by the leaky mode is higher compared to the modal gain of the light emitting device. The term "leaky mode" as used herein may be also referred to ae substrate radiation mode, and means s a mode generated when the conditions for total reflection of light power are not satisfied in the waveguide.

The modal gain gmax means a gain for a lasing mode and expressed by:

wherein a L, Rf, Rr, and T represent loss factor, length of the resonator, square of reflectance for the electric field at the front facet of the laser, square of the reflectance for the electric field at the rear facet is of the laser, and the light confinement factor of the lasing mode, respectively.

It was finally found after experiments that a semiconductor light emitting device having a lower waveguide loss can be achieved by sandwiching a quantum well active layer structure between a pair of SCH layers while in contact therewith, increasing the thickness of the first and second cladding layers compared to the conventional device to decrease the leaky mode waveguide loss down to 2s 3 cm 1 or less at the lasing wavelength.

Raoaivsd Oct-19-00 01 MB am From-0!B20B0BB3 To-Smart i Biggar Page 012 00-10-13:14:31 îttSffirFiM FAX03-523 G-08D3 ;03!329D08D3 * 13/ 21 m It is sufFicient- in fact that the leaky mode waveguide loss be below the waveguide loss generated by other causes in order to obtain the reduction of the threshold current for the lasing, although the leaky mode waveguide lose being lesa below 3 cm"1 was found in the experiments.

The waveguide loss was examined in relation to the compositions of the cladding layers and the SCH layers. It was found that the leaky mode equal to or below 3 cm"1 can be obtained by maintaining the light confinement factor of the waveguide layers including the RCH layers and the cladding layers at 90% or above.

The light power distribution P in the direction y perpendicular to the surface of the substrate is expressed by a Gaussian beam formula:

P=P0*exp (-y(y/W)2) wherein W is a half width of a range within which the light power assumes more than e"2 of the peak thereof, and P0 is a constant.

In order for maintaining 90% or above for the overall light confinement coefficient of waveguide layers including the quantum well active layers, the SCI1 layers and the cladding layers, the half width W, the thickness Ta of the SCH layers, the thickness Received Oet-13-00 01 i4Sain From-0!62Q60S63 To-Smart t Biggar Pag* 013 00-10-13:14:31 ifâft&tm FAX03-D29D-08D3 ;03523D08D3 « 14/ 2 1 lu Tclad of the cladding layer and the Lliickness Taot of the quantum, active layers should satisfy the following relationship:

Tact/2+Tsch+Telad > 2.3W.

In the above relationship, the constant 2-3 in the right side is determined for "k" so that the ratio of the integrate of the light power distribution P from -eo to +00 to Lhe integrate of P from -kW to +kW with respect to y assumes 0.9 (corresponding to 90% of the overall light confinement coefficient).

The half width W can be obtained by measuring the far field pattern (Fl'VP) of the light beam from the following expression:

W=(A/7r)/tan 9 wherein À is the lasing wavelength and 0 is a half width of the angle range within which the light power assumes above e"2 of the peak thereof when the FFP characteristics are depicted by a graph showing the light power plotted against the radiation angle.

In the above description, a facet emission semiconductor laser is exemplified. However, the present invention can be applied to a plane emission semiconductor laser, irrespective of the output wavelength of the semiconductor laser.

Received Oct-IS-OO 01:48ain Fron-0362060863 To-Smart t Biggar Page 014 00-10-13:14:31 ittSttitMffli FAX03-D2 3 5-08 D3 ;03D23S08!J3 # 15/ 2 1 Now, the present invention is more specifically described with reference to an embodiment of the present invention.

Referring to Fig. 1, a semiconductor light emitting device according to the embodiment of the present invention is implemented by an InGaAs based semiconductor laser of a facet emission type lasing at a wavelength of 980-nm range .

The semiconductor laser, generally designated by numeral 200, includes a TOO-At m-thick n-GaAs substrate 100, and epitaxial layers consecutively formed on the n-GaAs substrate 1 00. The epitaxial layers include, consecutively from the n-GaAs substrate 100, a 2-jttm-thick n-Alo.3Gao.7As cladding layer 110, a 30-nm-thick undoped Al02Gao.aAs SC11 layer 120, a quantum well structure 130 including a pair of undoped InGaAs/GaAs layers, a ijt0-nm-thick undoped Alo.zGan.sAs SCH layer 140, a 2- M in-thick p-Alo.3Gao.7As cladding layer 150 and a 0.3- U m- thick GaAs cap layer 160. The quantum well structure 130 and the pair of SCH layers 140 and 150 may be referred to as active layers. A single or plurality of pairs each including a quantum well layer and a barrier layer may be provided in the quantum well structure.

Raoeived Oct-13-00 01i48am From-03B20B08B3 To-Smart I Biggar Page 01E 00-10-13:14:31 iftaSSSM FAX03-D23D-08Q3 ;03529508D3 # 10/ 21 The cap layer 160 and the top portion of the cladding layer 150 are configured as a 4-At m-wide mesa stripe, and a passivation layer 170 made of SiN is formed on the side surfaces of the mesa « stripe and the exposed surface of the cladding layer 150. A p-side electrode 180 made of Ti/Pt/Au layered metallic films and an n-side electrode 190 made of AuGe/Ni/Au layered metallic films are formed on the passivation layer 170 and on the io bottom surface of the n-GaAs substrate 100, respectively.

Tn the present embodiment, the refractive index nni]h of the substrate 100 is 3.46 at the lasing wavelength, whereas the effective refractive index is iieff of the lasing waveguide mode is 3.35. In addition, the overall light confinement coefficient of the waveguide layers including the cladding layer 110, the active layers 130, and the cladding layer ISO is 99%, namely, above 90%. Further, the thickness of both the cladding layers 110 and 150 is 2 At m which is larger compared to the thickness 1.7 M m of the cladding layers in the conventional device.

In a practical sample of the semiconductor light emitting device experimentally fabricated for RecBived Oçt-13-QO 01:48ain From-03B29B08B3 To-Smart t Biggar Pags 016 m 00-10-13:14:37 ;tt3ffîF*M FAX03-D23 D-0803 ;03D2SD08D3 # 17/ 21 the present embodiment, the waveguide loss generated by the leaky mode was measured at below ÎJ cm"1. Fig. ÎJ shows the optical output of the sample measured against the injected current.

Tn Fig. 3, the threshold current of the sample is reduced to 30 mA compared to the threshold current of 35 mA for the conventional device including cladding layers having a thickness of 1.7 U m. In addition, the quantum efficiency was improved by 20%. The calculation of the waveguide loss in the sample and the conventional devices revealed that the conventional device including 1.7- A in-thick cladding layers involves a waveguide loss larger than 3 cm"1, whereas the waveguide loss of the sample having 2- //. m-thick cladding layers is below 3 cm"1. In short, the semiconductor emitting device of the present invention has a lower threshold current and a higher light emitting efficiency by lowering the waveguide loss.

Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention.

RocBived 0ct-1î-00 OliBBam F roin-0362060e63 To-Smart I Biggar Pags 001 00-10-13:14:37 ;t*w1wF*M FAX03-D29 5-08G3 ;03D29D08D3 # 18/ 21